UNIVERSITY  OF  CALIFORNIA 
FITMENT   OF   CIVIL    ENGINEERI.N 
BERKELEY.  CALIFORNIA 


UNIVERSITY  OF  CALIFORNIA 

DEPARTMENT  OF  CIVIL   ENGINEERING 

BERKELEY.  CALIFORNIA 


'•'"•••""'•  THE  ;    V: 

CHEMISTEY  OF  COLLOIDS 


PART  I 
KOLLOIDCHEMIE 

BY 

RICHARD  ZSIGMONDY 

Professor  at  the  University  of  Gottingen,  Director  of  the  Institute  of  Inorganic  Chemistry 

TRANSLATED    BY 

ELLWOOD  B.  SPEAR 

Associate  Professor  of  Inorganic  Chemistry,  Massachusetts  Institute  of  Technology 


PART  II 
INDUSTRIAL  COLLOIDAL  CHEMISTRY 

BY 

ELLWOOD  B.  SPEAR,  B.A.,  PH.D. 


A  CHAPTER  ON 

COLLOIDAL  CHEMISTRY  AND  SANITATION 

BY 

JOHN  FOOTE  NORTON,  S.B.,  PH.  D. 

Assistant  Professor  of  Chemistry  of  Sanitation,  Massachusetts  Institute  of  Technology 


FIRST  EDITION:  : 


NEW  YORK 
JOHN   WILEY   &   SONS,    INC. 

LONDON:    CHAPMAN  &  HALL,  LIMITED 
1917 


COPYRIGHT,  1917, 

BY 
ELLWOOD  B.  SPEAR 


Stanhope  jprcss 

F.    H.  G1LSON   COMPANY 
BOSTON,  U.S.A. 


G5- 


Engineering 
Library 


TRANSLATOR'S  PREFACE  TO  PART  I 

IN  Part  I  it  has  been  the  endeavor  to  give  an  accurate  translation 
of  the  German  edition.  The  translation  was  submitted  to,  and  ap- 
proved by  the  author. 

The  translator  wishes  to  acknowledge  herewith  valuable  and  pains- 
taking assistance  on  the  part  of  Edith  Taylor  Spear. 

ELLWOOD  B.  SPEAR. 

CAMBRIDGE,  MASS., 
March,  1917. 


in 


TABLE  OF  CONTENTS 
PART  I 

PAGE 

CHAPTER  I.  —  GENERAL  CONSIDERATIONS 3 

Definition  of  colloids.  Behavior  of  substances  toward  solvents.  Irrevers- 
ible colloids.  Colloids  and  their  significance.  Graham's  characteriza- 
tion of  colloids.  Definitions.  Coagulation.  Peptisation.  Differences 
between  solutions  of  irreversible  colloids  and  those  of  crystalloids.  Revers- 
ible and  irreversible  colloids.  Preparation  of  colloidal  solutions.  Ojg- 
tical '  properties.  Ultramicroscopy.  Polarization  by  small  particles. 
TyndalPs  phenomenon.  Determination  of  the  size  of  ultramicroscopical 
particles. 

CHAPTER  II.  —  CLASSIFICATION /?M<V^"  ^rYTXTT. . .' 19 

Change  of  properties  of  dispersed  systems  with  the  size  of  the  particles. 
Suspensions  and  hydrosols.  Coarse  suspensions.  Similarities  between 
colloidal  and  crystalloidal  solutions.  Behavior  of  hydrosols  on  evapora- 
tion. Behavior  of  specific  groups  toward  electrolytes.  Other  classifi- 
cations. Disperse  systems.  Disperse  systems  with  gas  as  disperse  medium. 
Disperse  systems  with  liquid  as  disperse  medium.  Disperse  systems  with 
solid  as  disperse  medium.  Classification  employed  in  this  book. 

CHAPTER  III.  —  PROPERTIES  OF  COLLOIDS 33 

Diffusion  and  osmotic  pressure.  Dialysis  and  ultrafiltration.  Apparatus 
for  dialysis.  Osmometer.  Movements  of  the  ultramicrons.  Brownian 
movement.  Electrical  properties.  Charge  on  the  particles.  Discharge 
of  the  particles.  Magnitude  of  the  charge  on  the  particles.  Coagulation. 
Rate  of  coagulation.  Valence  relations.  Absorption  and  adsorption.  Ad- 
sorption of  dissolved  crystalloidal  substances.  Adsorption  of  colloids  by 
other  substances.  Mutual  adsorption  of  ultramicrons.  Protection  effects. 
Influence  of  temperature  on  colloids.  Heat  of  colloidal  reactions.  Con- 
stitution of  jellies. 

CHAPTER  IV.  —  THEORY '. 71 

Theory  of  peptisation.  Nature  of  adsorbed  ions.  Behavior  of  hydrosols. 
Peptisation  of  stannic  acid  by  HC1.  Peptisation  of  other  colloids,  or  col- 
loidal combinations.  Transitional  stages  between  electrolytic  solutions 
and  irreversible  hydrosols.  Membrane  equilibria  in  the  presence  of  elec- 
trolytes. 

CHAPTER  V.  —  INORGANIC  COLLOIDS 86 

General  properties.  Pure  metal  colloids.  Conditions  of  stability.  Col- 
loidal gold.  Metallic  nature  of  colloidal  gold.  Theory  of  color.  Size  of 
particles  and  color  relations.  Absorption  spectra.  Polarization  by  the 
particles.  Change  of  color  and  coagulation.  Behavior  under  the  influence 


yi  TABLE  OF  CONTENTS 

PAGE 

of  a  fall  of  potential.  Reactions  of  colloidal  gold.  Protective  effect  and 
gold  number.  Theory  of  protective  action.  Colloidal  platinum.  Cata- 
lytic effect.  Poison  effects  on  platinum  sols.  Colloidal  silver.  Other 
colloidal  metals.  Protected  colloidal  metals.  Technical  colloidal  silver. 
Colloidal  palladium.  Colloidal  copper.  Other  metal  colloids. 

CHAPTER  VI.  —  COLLOIDAL  NONMETALS 129 

Colloidal  sulfur.     Colloidal  selenium. 

CHAPTER  VII.  —  COLLOIDAL  OXIDES 133 

Colloidal  silicic  acid.  Silicic  acid  gel.  Organogels  of  silicic  acid.  Struc- 
ture of  silicic  acid  gels.  Hydration  and  dehydration  of  silicic  acid  gels. 
Theory  of  capillarity  and  silicic  acid  gels.  Staining  of  silicic  acid  gels. 
Colloidal  stannic  acid.  Hydrosol  of  stannic  acid,  a  and  0  stannic  acid. 
Stannic  acid  gel.  Purple  of  Cassius.  Substances  analogous  to  Purple  of 
Cassius.  Colloidal  titanic  acid,  zirconium  oxide,  thorium  oxide.  Col- 
loidal metazirconic  acid.  Peptoids.  Colloidal  iron  oxide.  Iron  oxide  hy- 
drogels.  Colloidal  aluminium  and  chromium  oxides.  Colloidal  tung- 
stic  and  molybdic  acids.  Molybdenum  blue.  Other  colloidal  oxides. 

CHAPTER  VIII.  —  COLLOIDAL  SULFIDES - 176 

Colloidal  arsenious  sulfide.     Other  sulfide  hydrosols. 

CHAPTER  IX.  —  COLLOIDAL  SALTS 179 

Colloidal  silver  halides.  Photohaloids.  Colloidal  ferrocyanides.  Other 
colloidal  salts. 

CHAPTER  X.  —  ORGANIC  COLLOIDS 188 

Colloidal  organic  salts.  Soaps.  Effect  on  the  boiling  point.  Detergent 
action.  Emulsification  of  the  fatty  series. 

CHAPTER  XI.    DYESTUFFS 193 

Ultramicroscopy  and  dialysis  of  dyestuffs.  Composition  and  colloidal 
character.  Conductivity  of  solutions  of  dyestuffs.  Influence  of  electro- 
lytes. Protective  effect  and  dyestuffs.  Dyeing.  Capillary  analysis. 

CHAPTER  XII.  —  PROTEIN  BODIES 208 

Classification.  Albumins.  Globulins.  Compound  proteids  and  albu- 
minoids. Decomposition  products  of  protein  bodies.  General  behavior  of 
protein  bodies.  Precipitation,  with  electrolytes.  Neutral  proteids. 
Electric  charge.  Coagulation  and  denaturization  of  proteids.  Behavior 
of  globulins.  Gelatins,  preparation  and  properties.  Haemoglobin  and 
oxyhaemoglobin.  Molecular  weight  determinations.  Casein.  Occurrence 
in  milk.  Acid  and  rennet  coagulation. 


PART  H 

CHAPTER  XIII.  —  INTRODUCTION  TO  PART  II 241 

Colloids  and  amorphous  solids.  General  formula  applicable  to  colloidal 
particles,  Un.  Explanation  of  U  and  N.  Different  kinds  of  coagulation. 
Methods  of  growth  of  particles  in  solution.  Reversible  and  irreversible 
coagulation. 


TABLE  OF  CONTENTS  vii 

PAGE 

CHAPTER  XIV.  —  SMOKE,   FLUE  FUMES,  LIQUID  PARTICLES  IN  GASES.    243 
The  smoke  nuisance  and  its  prevention.     Flue  fumes  and  methods  of  pre- 
cipitation.    Washing.     Centrifugalizing.     Settling  chambers.     Use  of  baf- 
fle plates.     Filtering.    Electrical  precipitation,  Cottrell  process. 

CHAPTER  XV.  —  RUBBER f 251 

The  latex  and  its  composition.  Charge.  Brownian  movement.  Source. 
Size  of  particles.  Methods  of  coagulation.  Precipitating  agents.  Vulcani- 
zation cold  and  hot.  Theories  of  vulcanization.  Chemical  theory.  Ad- 
sorption theory.  Regeneration  of  rubber. 

CHAPTER  XVI.  —  TANNING 262 

Cell  and  cell  walls  of  the  raw  hide.  Rehydration  and  swelling.  Function 
of  tanning  agent.  Chemical  and  adsorption  reactions.  Mineral  tanning. 
Effect  of  protective  colloids  on  tanning  solutions. 

CHAPTER  XVII.  —  MILK 264 

Disperse  phases  in  milk.  Size  of  fat  particles.  Coagulation  of  fat  par- 
ticles. Homogenization,  and  its  effect  on  fat  particles.  Making  of  ice 
cream. 

CHAPTER  XVIII.— COLLOIDAL  GRAPHITE 266 

Colloidal  graphite.     Oil-  and  waterdag.     Deflocculation  of  graphite. 

CHAPTER  XIX.  —  CLAYS 267 

Clays.  Composition.  Colloidal  matter.  Dehydration  and  rehydration. 
Adsorption  by  clays.  Reactions  caused  by  adsorption.  Deflocculation  of 
clays.  Deflocculants.  Soils.  Humus  and  its  function.  Action  of  lime 
on  clay  and  humus. 

CHAPTER  XX.  —  COLLOIDS  IN  SANITATION 270 

Colloidal  nature  of  bacteria.  Charge.  Nature  of  colloids  found  in  water. 
Natural  precipitation.  Artificial  precipitation.  Slow  sand  filtration. 
Rapid  filtration,  the  "American"  method.  Colloidal  reactions  involved. 
Sewage  purification.  Chemical  precipitation.  Intermittent  sand  filtra- 
tion. Activated  sludge  process.  Colloidal  chemistry  of  disinfection. 

INDEX  . ,  275 


PART  I 


CHEMISTRY  OF  COLLOIDS 


CHAPTER  I 
GENERAL  CONSIDERATIONS 

Definition  of  Colloids 

IN  an  article  published  in  Philosophical  Transactions  in  1861, 
Graham  *  differentiated  between  crystalloidal  and  colloidal  sub- 
stances. To  the  first  of  these  two  classes  belong  those  substances 
which  exhibit  an  appreciable  rate  of  diffusion  and  possess  the  prop- 
erty of  penetrating  membranes  made  of  parchment  paper.  Accord- 
ing to  Graham  the  second  group,  colloids,  comprises  those  substances 
which  have  no  marked  tendency  toward  diffusion,  nor  toward  the 
penetration  of  membranes  or  gels. 

Because  of  a  large  number  of  facts  that  have  been  discovered  from 
time  to  time  this  classification  can  no  longer  be  retained.  On  the 
one  hand  there  are  a  great  number  of  transitions  between  colloids 
and  crystalloids,  so  that  a  sharp  line  can  scarcely  be  drawn  between 
the  two  domains.  This  difficulty,  however,  exists  in  many  classifica- 
tions other  than  that  of  Graham's.  On  the  other  hand  there  are 
many  substances  that  will  form  colloidal  or  crystalloidal  solutions 
depending  upon  the  solvent  employed.  A  classical  example  of  this 
is  sodium  stearate,  which  has  been  studied  by  Krafft  f  and  found  to 
be  a  true  crystalloid  in  alkaline  solution,  while  it  exhibits  colloidal 
properties  in  neutral  solution.  Other  alkali  salts  of  the  higher  fatty 
acids  behave  similarly. 

From  these  and  numerous  other  examples  it  follows  that  Graham's 
classification  relates  to  mixtures  rather  than  to  chemically  pure  sub- 
stances. A  discussion  of  colloids  from  this  point  of  view  would  have 
to  do  not  with  pure  substances  and  their  properties,  but  rather  with 
those  mixtures  of  substances  which  have  colloidal  properties.  Ordi- 
nary chemical  terminology  also  justifies  this  standpoint  in  that  by  col- 
loidal silicic  acid,  gold,  or  platinum,  for  example,  we  understand  not 

*  Th.  Graham:  Philos.  Transact.,  183  (1861);  Liebigs  Annalen,  121, 1-77  (1862). 
t  See  Chapter  X. 

3 


CHEMISTRY   OF  COLLOIDS 

the  pure  substances,  but  rather  mixtures  of  silicic  acid,  gold,  or  plati- 
num with  water  or  some  other  medium;  in  other  words.,  we  mean  colloi- 
dal solutions  of  these  substances,  or  mixtures  of  them  with  protective 
colloids.  Similarly  gels,  which  are  never  chemical  individuals,  but 
are  always  mixtures  of  at  least  two  substances,  are  classed  as  colloids. 
It  is  in  general  more  exact  to  speak  of  colloidal  systems  than  of  col- 
loidal substances,  and  to  understand  by  the  term  "  colloid  "  a  colloidal 
system. 

Nevertheless  many  pure  substances  may  be  regarded  as  crystal- 
loids or  colloids  according  to  Graham's  classification,  if  we  but  take 
into  consideration  that  some  of  them  when  brought  into  contact 
with  a  given  solvent  always  form  a  crystalloidal  solution,  while  others 
invariably  show  colloidal  properties  in  solution. 

Behavior  of  Substances  Toward  Solvents 

If  the  behavior  of  dry  substances  toward  solvents  is  considered  the 
following  cases  may  be  distinguished: 

1.  Many  are  either  insoluble  in  the  given  solvent,  or  they  dissolve 
spontaneously  and  form  without  exception  crystalloidal   solutions, 
e.g.,  sugar  or  sodium  chloride  in  water,  benzoic  acid  in  alcohol,  naph- 
thalene in  benzol,  gold  in  mercury. 

2.  Others  when  brought  into  contact  with  a  given  solvent  remain 
undissolved,  or  they  form  spontaneously  colloidal  solutions  only,  e.g., 
hemoglobin,   albumin,   dextrin  in  water;    caoutchouc   in   benzol  or 
carbon  disulfide;    resinates  in  ethereal  oils.     Substances  of  this  sort 
even  in  a  pure  state  can,  without  question,  be  considered  as  colloids 
according  to  Graham's  classification. 

3.  There  are,  however,  as  already  pointed  out,  some  substances 
that  occupy  a  position  intermediate  between  classes  one  and  two,  in 
that  they  form  crystalloidal  solutions  with  some  solvents  and  colloi- 
dal with-  others.     In  most  cases  of  this  sort  the  formation  of  a  col- 
loidal solution  is  caused  by  a  chemical  reaction  between  the  substance 
in  question  and  the  solvent,  whereby  a  compound  is  formed  that  is 
practically  insoluble  in  the  surrounding  medium. 

Irreversible  Colloids.  —  It  is  of  great  importance  to  note  that  sub- 
stances falling  into  class  1  can  also  be  obtained  in  colloidal  solution 
provided  that  such  media  are  chosen  in  which  the  substance  does  not 
spontaneously  dissolve;  that  is  in  which  it  is  practically  insoluble. 
This  end  can  be  arrived  at  by  the  employment  of  electrical  energy,  e.g., 
Bredig's  and  Svedberg's  methods  for  the  preparation  of  colloidal 
metals.  Or  the  same  end  may  be  achieved  by  means  of  chemical 
reactions  whereby  the  substance  is  formed  in  a  liquid  in  which  it  is 


GENERAL  CONSIDERATIONS  5 

insoluble,  e.g.,  the  preparation  of  colloidal  gold,  silicic  acid,  arsenious 
sulfide,  silver  iodide,  etc.  Naturally  in  the  preparation  of  colloidal 
solutions  of  this  sort  certain  well-defined  procedures  must  be  closely 
followed  in  order  to  obtain  the  substance  in  a  finely  divided  state, 
and  to  prevent  its  immediate  precipitation.  Attention  should  be 
called  here  to  the  fact  that  colloidal  solutions  of  this  latter  variety 
if  they  are  pure,  that  is  uncontaminated  by  the  presence  of  any  be- 
longing to  class  2,  behave  on  being  dried  in  a  fundamentally  different 
manner  from  those  belonging  to  class  2.  For,  while  the  colloids  of 
class  2  on  drying  leave  a  residue  that  will  in  general  form  the  original 
colloidal  solution,  if  brought  in  contact  with  the  solvent,  those  in 
class  3  go  through  a  series  of  irreversible  changes  of  state  during  the 
drying  process.  As  a  result  the  residue  has  lost  the  property  of  again 
spontaneously  forming  the  original  colloidal  solution  with  the  liquid 
from  which  the  residue  was  obtained.  Colloids  of  class  2  are  known 
as  reversible,  those  of  class  3  as  irreversible,  colloids.* 

Colloids  and  Their  Significance 

In  order  to  estimate  the  significance  of  colloidal  chemistry  it  is 
necessary  to  introduce  a  number  of  substances,  or  systems  of  sub- 
stances, that  belong  to  colloids.  In  nature  they  are  very  widely  dis- 
tributed. All  living  matter,  animal  as  well  as  vegetable,  is  for  the 
most  part  built  up  of  colloids.  Without  colloids  life  is  impossible. 
Cells,  their  contents  and  membranes,  consist  of  colloids.  Blood  serum 
and  the  sap  of  plants  are  intrinsically  colloidal  solutions.  Glue, 
which  is  obtained  from  leather  and  bones,  is  a  typical  colloid.  Hemo- 
globin, the  red  coloring  matter  in  blood;  rubber  and  guttapercha  that 
flow  from  trees;  vulcanized  caoutchouc;  starch  and  its  by-product, 
dextrin;  cellulose  and  its  nitric  acid  ester,  the  explosive  nitrocellu- 
lose; collodion,  silk,  wool,  artificial  silk,  etc.,  are  all  colloids.  Most 
of  these  are  not  simple  substances  but  are  none  the  less  important 
for  natural  processes  or  for  the  industries.  Furthermore  the  principal 
articles  of  man's  food  consist  of  colloids;  proteids  are  colloids  par 
excellence. 

In  the  domain  of  inorganic  chemistry  also  colloids  are  frequently 
met  with,  and  although  they,  are  not  so  important  as  in  the  field  of 
organic  chemistry,  especially  that  of  organisms,  nevertheless  the  study 
of  these  colloids  greatly  advanced  our  knowledge  of  the  subject, 
chiefly  because  they  lend  themselves  much  better  to  investigation  on 
account  of  the  simplified  conditions  under  which  they  can  be  obtained. 

*  W.  B.  Hardy:  Zeit.  f.  phys.  Chemie,  33,  326,  385  (1900);  Biltz:  Ber.,  37,  1096 
(1904);  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  20  (1905). 


6  CHEMISTRY  OF  COLLOIDS 

In  farming  and  various  industries  colloids  play  a  very  important 
part.  According  to  van  Bemmelen  *  it  is  the  colloids  in  the  soil  that 
retain  the  food  of  plants  and  subsequently  give  it  up  to  them.  Fer- 
tilization with  potassium  carbonate  and  soluble  phosphates,  with 
saltpeter  and  ammonium  salts  would  scarcely  have  any  point  for  agri- 
culture unless  these  salts  were  adsorbed  by  the  colloids  in  the  soil, 
and  continuously  imparted  to  the  roots  of  the  plants.  Colloids  have 
a  very  important  bearing  on  the  industries.  As  already  indicated,  a 
large  number  of  the  raw  materials  as  well  as  the  finished  products 
belong  to  the  field  of  colloidal  chemistry.  Many  of  the  reactions  in- 
volved in  ceramics,  glass  making,  dyeing,  and  certain  other  industries 
can  be  thoroughly  understood  only  with  a  fundamental  knowledge  of 
colloidal  chemistry.  Moreover  the  study  of  colloidal  chemistry  has 
thrown  considerable  light  on  the  cement  and  lime  industries.  The 
behavior  of  waste  liquors  from  factories  as  far  as  the  colloidal  con- 
tents are  concerned  has  become  intelligible  through  investigations 
along  the  lines  of  colloidal  chemistry.  These  investigations  have  re- 
sulted in  new  methods  for  dealing  with  the  problems  that  arise  in 
this  field.  Liippo-Cramer  f  has  pointed  out  that  colloidal  chemistry 
is  responsible  for  an  important  awakening  in  photography.  In  physi- 
ology, in  chemistry,  in  pharmacy,  and  in  immuno-chemistry  J  colloids 
have  a  prominent  place.  Indeed  in  almost  every  domain  the  new 
views  with  regard  to  colloidal  chemistry  are  becoming  general. 

Graham's  Characterization  of  Colloids 

Because  of  the  very  wide  distribution  of  colloids  it  is  natural  for 
investigators  in  almost  all  branches  of  science  to  turn  their  attention 
in  the  direction  of  the  study  of  these  substances.  Botanists,  zoolo- 
gists, physicians,  and  physiologists  side  by  side  with  physicists  and 
chemists  have  engaged  in  this  work,  and  through  the  efforts  of  these 
considerable  progress  has  been  made.  It  is  but  necessary  to  call  to 
mind  the  names  Nageli,  Butschli,  van  Bemmelen,  Quincke,  Hardy, 
Henri  to  show  how  many-sided  are  the  problems  presented  in  the 
field  of  colloidal  chemistry. 

To  a  chemist,  Thomas  Graham,  §  belongs  fhe  credit  of  being  the 
first  to  recognize  and  describe  in  detail  the  properties  common  to 
colloids.  Although  the  two  classes  of  solutions,  colloidal  and  crystal- 

*  J.  M.  van  Bemmelen:  Landw.  Vers.-Stat.,  36,  69-136  (1888). 
t  Liippo-Cramer:  Kolloidchemie  imd  Photographic.     Dresden  (1908). 
I  H.  Bechhold:  Die  Kolloide  in  Biologic  und  Medizin.     Dresden  (1912). 
§  See  page  3. 


GENERAL  CONSIDERATION'S  7 

loidal,  are  closely  associated  by  transitions,  as  pointed  out  on  page  3, 
Graham's  classification  was  the  first  to  give  a  comprehensive  survey 
of  the  differences  between  them.  Here,  as  in  many  other  cases,  prog- 
ress was  coincident  with,  and  dependent  upon,  the  discovery  of  new 
methods.  During  his  experiments  Graham  noticed  striking  differ- 
ences in  the  rate  of  diffusion  of  various  substances.  He  found  that 
alkalis,  acids,  and  salts,  as  well  as  sugar  and  alcohol,  all  crystalloids, 
diffused  very  rapidly  compared  with  the  slow  rate  of  other  closely 
allied  substances  which  he  called  "  fixed."  The  former  he  compared 
to  volatile,  the  latter  to  less  volatile,  substances.  Substances,  such  as 
dextrin,  rubber,  caramel,  proteids,  glue,  etc.,  exhibited  the  further 
peculiarity  of  not  (or  at  least  with  extreme  slowness)  diffusing  through 
gels,  plant  or  animal  membranes.  Because  glue  shows  these  and  other 
properties  common  to  colloids  in  an  especially  noticeable  manner, 
Graham  called  this  class  of  substances  "  colloids  "  after  glue  (Greek, 
kolla  —  glue,  and  eidos  —  appearance).  Crystalloids,  such  as  sugar, 
table  salt,  etc.,  on  the  other  hand,  pass  through  gels  or  membranes 
with  extraordinary  ease. 

Graham  based  his  method  of  separating  colloids  from  the  accom- 
panying crystalloidal  substances  upon  the  differences  in  the  rate  of 
diffusion.  This  method  he  called  dialysis.  His  apparatus,  the  dia- 
lyzer,  is  very  simple.  Moistened  parchment  membrane  is  brought 
over  the  edge  of  a  cylindrical  hard  rubber  ring,  and  bound  with  a 
string,  thus  forming  a  dish  as  shown  in  Fig.  1.  The  whole  floats  in  a 
dish  containing  distilled  water.  The 
solution  to  be  dialyzed  is  placed  in  the 
dish  made  of  the  membrane  and  the 
rubber  ring,  and  allowed  to  float  on 
the  surface  of  the  distilled  water  for 
some  time.  If  one  subjects  to  dialysis 
a  mixture  of  sugar  and  gum  arabic, 
both  dissolved  in  water,  the  sugar  dif-  FlG'  L  Graham's 
fuses  gradually  into  the  outer  vessel  and  the  gum  arabic  remains 
behind.  It  is  now  only  necessary  to  change  the  water  frequently 
and  to  allow  the  dialysis  to  continue  until  no  more  crystalloids  (in 
this  case  sugar)  pass  from  the  inner  to  outer  vessel.  If  one  begins 
with  a  suitable  mixture,  e.g.,  a  solution  of  sodium  silicate  saturated 
with  hydrochloric  acid,  it  is  possible  to  obtain  the  colloid  in  a  fairly 
pure  state,  the  colloid  being,  in  this  case,  silicic  acid.  Small  amounts 
of  electrolytes  almost  always  remain  behind,  the  significance  of  which 
we  shall  deal  with  later  on. 


8  CHEMISTRY  OF  COLLOIDS 

Definitions 

Colloidal  solutions  obtained  in  this  manner  with  water  as  a  solvent 
Graham  called  "  hydrosols."  If  alcohol  is  used  instead  of  water  alco- 
sols  are  obtained.  In  general  colloidal  solutions  where  organic  sol- 
vents are  employed  are  called  organosols.  Colloidal  solutions  are 
frequently  designated  by  the  term  sol.  A  noteworthy  property  of 
many  colloidal  solutions  is  the  tendency  to  form  a  jelly-like  half  solid 
mass,  when  a  large  portion  of  the  solvent  is  removed,  or  when  the 
solution  is  subjected  to  the  influence  of  salts  or  other  foreign  bodies. 
From  hydrosols  hydro:-els,  from  alcosols  alcogels,  are  obtained.  On 
further  drying  the  apparently  amorphous  solid  residue  may  be  trans- 
parent and  glassy,  crumbling  and  porous,  or  in  the  form  of  a  powder. 
These  largely  dehydrated  residues  are  usually  referred  to  as  hydro- 
gels,  or,  more  generally,  gels,  even  if  they  no  longer  have  the  property 
of  becoming  spontaneously  dispersed  in  the  original  solvent.  The 
dried  residues  of  reversible  colloids  are  called  solid  sols  by  Lotter- 
moser.* 

Coagulation.  —  Coagulation,  the  change  of  concentrated  irreversi- 
ble hydrosols  to  a  jelly-like  state,  often  occurs  spontaneously.  This 
process  closely  resembles  the  crystallization  of  a  very  soluble  salt  from 
a  supersaturated  solution  by  inoculation  with  a  tiny  crystal.  Fre- 
quently the  process  takes  place  slowly,  allowing  the  coagulation  to  be 
followed  step  by  step.  Coagulation  may  occur  in  dilute  solution  also 
and  in  these  cases  amorphous  precipitates  are  obtained  instead  of 
jellies.  At  the  temperature  of  the  coagulation  these  amorphous  pre- 
cipitates are  not  soluble  in  an  excess  of  the  solvent,  or  at  least  to  a 
very  limited  extent.  Coagulation  is  therefore  an  irreversible  change 
of  state  of  colloids. 

Peptisation.  —  Under  the  influence  of  sometimes  very  small  quan- 
tities of  foreign  substances  hydrogels  may  be  changed  into  hydrosols. 
Having  in  mind  the  formation  of  peptones  from  proteids  by  means  of 
HC1,  Graham  called  this  reaction  peptisation.f  Many  differences 
between  the  dissolving  of  colloids  and  crystalloids  are  easily  discern- 
ible. If  a  crystal  is  dipped  into  a  solvent  in  which  it  will  dissolve, 
for  instance  NaCl  in  water,  it  will  be  seen  that  the  crystal,  without 
taking  up  any  of  the  solvent,  gives  its  outside  layers  to  the  surround- 
ing liquid.  At  any  moment  the  residual  piece  has  the  exact  compo- 
sition of  the  original  crystal.  The  analogy  to  the  evaporation  of  a 
volatile  substance  is  far  reaching  and  has  long  been  recognized.  Re- 

i     *  A.  Lottermoser:  Uber  anorganische  Kolloide,  2.    Stuttgart  (1901). 

f  Graham:  Proc.  Roy.  Soc.,  16,  Jimp  (1864);  Poggendorffs  Annalen,  123,  529-541 
(1864). 


GENERAL  CONSIDERATIONS  9 

versible  colloids,  on  the  other  hand,  behave  in  quite  another  manner 
toward  solvents.  Particles  of  the  solid  are  given  up  to  the  liquid,  it 
is  true,  but  the  solid  also  takes  up  considerable  amounts  of  the  liquid. 
Colloids  distend  before  they  dissolve  and  this  swelling  process  has  in 
many  cases  been  closely  studied.  The  gels  of  many  reversible  col- 
loids formed  by  swelling,  gelatin  for  instance,  dissolve  only  on  raising 
the  temperature;  others  go  into  solution  at  room  temperature,  e.g., 
gum  arabic. 

The  reverse  processes  on  evaporation  follow  similar  lines.  From 
a  sufficiently  concentrated  solution  of  NaCl  crystals  separate  out 
directly  and  have  the  same  composition  as  the  original  substance 
dissolved  to  form  the  solution.  On  the  other  hand  solutions  of  glue, 
rubber,  etc.,  during  evaporation  pass  through  several  intermediate 
steps  between  the  liquid  and  solid  states,  before  they  finally  become 
congealed.  In  a  solid  state  colloids  always  retain  a  portion  of  the 
solid  or  medium,  but  never  in  stoichiometric  relations  such  as  exist 
between  crystals  and  their  water  of  crystallization.  We  see,  there- 
fore, that  crystalloids  pass  directly  from  the  solid  to  the  liquid,  and 
back  again  from  the  liquid  to  the  solid  state,  while  colloids  go  through 
a  gradual  change,  involving  innumerable  intermediate  steps.  At 
this  point  a  misconception  for  which  Graham's  terms,  crystalloid  and 
colloid,  are  responsible,  should  be  called  to  the  attention  of  the  reader. 
Graham  rightly  remarked  that  colloidal  solutions  generally  leave 
behind  an  amorphous  (better  amorphous  appearing)  mass,  or  residue. 
From  this  it  is  sometimes  concluded  that  colloids  never  crystallize. 
This  is  not  correct,  for  when  certain  precautions  are  taken  crystals 
may  be  caused  to  grow  in  many  colloidal  solutions.  Crystallized 
albumin,  globulin,  hemoglobin,  etc.,  exist,  and  even  from  solutions 
of  colloidal  silver  crystals  of  silver  may  be  obtained.  Furthermore 
the  amorphous  appearing  residues  may  be  in  no  way  truly  amorphous, 
but  rather  consist  of  ultramicroscopic  crystals  that  appear  to  be 
amorphous  merely  because  the  microscope  reveals  aggregations  of 
crystals  and  not  the  individuals. 

Differences  Between  Solutions  of  Irreversible  Colloids  and 
those  of  Crystalloids 

The  preparation  of  irreversible  hydrosols  indicates  that  they  differ 
in  many  respects  from  solutions  of  crystalloids.  For,  while  the  latter 
are  obtained  directly  by  dissolving  the  solute  in  the  solvent,  round- 
about methods  are  necessary  in  the  former  case  to  procure  a  state  of 
fine  subdivision.  In  point  of  fact  closer  investigations  have  shown 
that  irreversible  hydrosols  do  not  in  general  exist  in  a  state  of  molec- 


10  CHEMISTRY  OF  COLLOIDS 

ular  dispersion  as  is  the  case  with  crystalloids,  but  that  in  the  solu- 
tion all  possible  sizes  of  particles  occur  from  the  molecular  to  those 
of  microscopical  dimensions.  These  differences  were  noticed  at  an 
early  stage  and  the  endeavor  was  made  to  characterize  both  rever- 
sible and  irreversible  hydrosols  as  suspensions,  in  contradistinction 
to  the  unquestionably  homogeneous  crystalloidal  solutions.  In  the 
opinion  of  the  author  this  term  is  not  suitable  to  characterize  accurately 
nor  correctly  the  field  with  which  we  are  concerned.  It  is  based  on  a 
restricted  point  of  view,  and  does  not  take  into  consideration  the  fact 
that  the  properties  of  matter  undergo  considerable  change  when  the 
particles  are  further  subdivided.  Moreover  it  is  not  in  accord  with 
the  common  usage  among  chemists. 

Reversible  and  Irreversible  Colloids 

The  fact  that  both  reversible  colloids  and  crystalloids  go  into  solu- 
tion spontaneously  would  lead  us  to  believe  that  they  are  closely 
related,  and  differ  probably  only  in  the  matter  of  molecular  weight. 
Graham  expressed  this  idea,  and  many  others  have  the  same  point 
of  view.  It  is,  however,  only  a  part  of  the  truth.  In  point  of  fact 
there  are  certain  colloids  where  there  are  good  grounds  for  assuming 
that  the  subdivision  during  solution  reaches  molecular  dimensions, 
but  the  particles  are  so  large  that  they  cannot  pass  through  parch- 
ment or  collodion  membranes.  Hemoglobin  of  cattle  is  an  example; 
the  molecular  weight  was  found  by  three  methods  to  be  in  the  neigh- 
borhood of  16,500.*  The  methods  are  given  on  page  233.  Another 
instance  similar  to  this  is  that  of  soluble  starch. 

In  general  large  molecules  have  the  tendency  of  uniting  to  form 
still  larger  aggregates.  This  tendency,  which  is  so  in  evidence  in  the 
formation  of  gels,  nature  makes  use  of  in  the  building  up  of  animal 
tissues.  Animal  fluids,  such  as  blood,  milk,  etc.,  contain  in  addition 
to  the  microscopical  blood  corpuscles,  fat  particles,  etc.,  a  myriad  of 
much  smaller  bodies  that  can  be  distinguished  by  means  of  the  ultra- 
microscope.  Solutions  of  many  reversible  colloids,  such  as  globulin, 
gelatin,  and  many  solutions  of  natural  and  artificial  coloring  matters, 
are  filled  with  submicroscopical  particles  that,  in  the  main,  are  larger 
than  those  found  in  gold  hydrosols.  In  characterization  of  reversible 
colloids  it  is  necessary  to  add  that  irreversible  colloids  may  become 
reversible  by  the  addition  of  other  reversible  colloids. 

From  what  has  been  said  it  cannot  be  concluded  that  solutions  of 
reversible  colloids  more  nearly  approach  the  crystalloidal  condition 
than  do  those  of  the  irreversible,  although  certain  dry  residues  of  the 

*  See  Chapter  XII,  page  233. 


GENERAL  CONSIDERATIONS  11 

former  have  the  property  of  forming  the  original  sol  with  the  solvent. 
Neither  is  it  in  accordance  with  the  facts  to  imagine  a  greater  similarity 
between  suspensions  and  irreversible  colloids  than  between  suspensions 
and  reversible;  nor  is  it  true  that  irreversible  colloids  contain  larger 
particles  than  the  reversible.  On  the  contrary  it  is  theoretically 
possible  to  prepare  irreversible  hydrosols  with  greater  dispersion,  or 
homogeneity,  than  exists  in  solutions  of  some  of  the  reversible  colloids, 
even  when  the  subdivision  of  the  latter  reaches  molecular  dimensions 
as  in  the  case  of  hemoglobin  from  the  blood  of  cattle. 

Preparation  of  Colloidal  Solutions 

Colloidal  solutions  may  be  made  by  many  different  methods.  The 
least  difficult  of  these  is  the  preparation  of  the  hydrosol  of  a  reversible 
colloid.  If  the  dry  colloid  in  question  is  brought  in  contact  with  the 
medium,  or  solvent,  the  dissolution  occurs  spontaneously.  In  this 
way  colloidal  solutions  of  rubber,  dextrin,  proteids,  molybdic  acid, 
tungstic  acid,  Lea's  colloidal  silver,  PaaPs  colloidal  gold,  and  many 
others  may  be  obtained.  Hydrosols  may  be  made  from  the  hydrogels 
of  irreversible  colloids  by  peptisation,  provided  that  the  dehydration 
has  not  been  carried  too  far;  in  which  case  the  new  colloidal  solution 
may  be  obtained  only  by  the  employment  of  roundabout  ways.  In 
the  case  of  metals  colloidal  solutions  are  often  made  by  electrical 
colloidation  (sparking,  atomization),  while  others  are  obtained  by 
chemical  means.  Thus,  dehydrated  silicic  acid  may  be  changed  into 
a  soluble  silicate  by  fusing  with  alkali.  The  aqueous  solution  of 
this  is  treated  with  HC1,  and  subsequent  dialysis  yields  the  corre- 
sponding hydrosol.  Hydrosols  of  metals  can  also  be  prepared  by  dis- 
solving the  metal  in  a  suitable  acid,  and  carefully  reducing  the  com- 
pound according  to  well-defined  procedures.  Methods  of  this  sort 
may  be  called,  according  to  Svedberg,*  condensation  methods,  while 
the  electrical  colloidation  according  to  Bredig  or  Svedberg  is  a  disper- 
sion method.  The  preparation  of  specific  colloids  will  be  taken  up 
more  fully  in  the  chapters  comprising  the  special  part  of  this  work. 

Optical  Properties 

Well-prepared  colloidal  solutions  are  transparent  or  slightly  opal- 
escent, and  may  be  either  colored  or  colorless.  They  readily  pass 
through  filter  paper  without  leaving  a  residue.  Every  cubic  milli- 
meter of  a  definite  colloidal  solution  has  identical  properties  with, 
and  gives  precisely  the  same  reactions  as,  every  neighboring  portion. 

*  The  Svedberg:  Die  Methoden  zur  Herstellung  kolloider  Losungen  anorganischer 
Stoffe.  Dresden  (1909). 


12  CHEMISTRY  OF  COLLOIDS 

Within  macroscopical  limits  colloidal  solutions  are  homogeneous,  and 
appear  so  under  the  ordinary  microscope.  Nevertheless  almost  all 
colloidal  solutions  are  found  not  to  be  homogeneous  if  investigated 
by  means  of  the  Tyndall  effect.  This  inhomogeneity  can  vary  mark- 
edly in  different  solutions  of  one  and  the  same  colloid  without  involv- 
ing a  corresponding  change  of  properties  or  chemical  reactivity.  In 
fact  it  is  possible  to  obtain  with  one  and  the  same  substance  either 
hydrosols  in  which  the  optical  inhomogeneity  can  scarcely  be  observed 
or  those  that  possess  a  distinct  turbidity. 

The  ultramicroscope  affords  an  opportunity  for  the  closer  examina- 
tion of  the  nature  of  the  diffusion  of  light  rays  by  hydrosols,  and  gives 
a  glimpse  into  the  spacial  discontinuity  in  these  systems.  The  mere 
presence  of  this  discontinuity  could  be  foretold  by  the  Tyndall  effect, 
but  it  remained  for  the  ultramicroscope  to  reveal  the  fact  that  colloi- 
dal solutions  contain  small  individual  particles  in  rapid  motion,  the 
size  of  which  particles  is  somewhat  larger  than  the  molecules  in  crystal- 
loidal  solutions.  The  investigation  of  hydrosols  with  this  instrument 
has  made  possible  much  more  definite  ideas  with  regard  to  the  size, 
color,  polarization,  motion,  etc.,  of  these  particles,  and  permitted  the 
scientist  to  enter  a  field  that  had  been  practically  closed  to  him. 
However,  some  of  the  particles  present  in  the  hydrosols  are  so  small 
that  they  cannot  be  detected  even  by  the  ultramicroscope,  and  this 
led  to  a  further  subdivision  of  particles  according  to  whether  or  not 
they  could  be  seen  by  this  means. 

Ultramicrons.  —  According  to  the  nomenclature  suggested  by  Sieden- 
topf,*  particles  smaller  than  the  resolving  power  of  the  ordinary 
microscope  are  called  ultramicroscopic,  regardless  of  whether  they  are 
visible  under  the  ultramicroscope  or  not.  These  particles  are  called 
submicroscopical  if  they  can  be  seen,  and  amicroscopical  if  they  can- 
not be  seen  by  the  ultramicroscope.  According  to  the  suggestion  of 
the  author,f  submicroscopical  particles  are  known  as  submicrons, 
and  amicroscopical  as  amicrons. 

Colloidal  solutions  of  metals  lend  themselves  particularly  well  for 
ultramicroscopical  examination  because  of  the  marked  difference  in 
the  optical  constants  of  the  disperse  phase  and  the  disperse  medium. 
It  is  possible  to  render  the  particles  in  ruby  glass  visible  although  they 
are  only  about  6  /*/*  in  diameter.  The  limits  of  visibility  are  reached 
much  sooner  in  the  case  of  colloidal  oxides,  and  as  a  general  rule  in  the 
organic  colloids  also.  Ultramicroscopical  particles  of  such  hydrosols 
become  amicrons  at  a  diameter  of  about  30-40  MM- 

*  H.  Siedentopf :  Berl.  klin.  Wochenscher,  Nr.  32  (1904). 
t  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  87  (1905). 


GENERAL  CONSIDERATIONS 


13 


Ultramicroscopy 

Ultramicroscopical  particles  of  not  too  small  diameter  can  be  ren- 
dered visible  by  a  simple  method  which  has  been  described  by  the 
author  in  his  publication,  "  Zur  Erkenntnis  der  Kolloide."  For  very 
small  submicrons  a  good  ultramicroscope  is  essential.  Fig.  2  is  a 
diagram  of  the  ultramicroscope  according  to  Siedentopf  and  Zsig- 
mondy.* 

The  source  of  light  is  an  arc  lamp,  or,  still  better,  the  sun.  The 
telescope  objective  FI  throws  an  image  of  the  source  of  light  on  the 
bilateral  slit  S.  A  second  telescope  objective  F2,  having  a  focus  of 


Scale  1:20 

FIG.  2.    Slit  ultramicroscope. 

about  80  mm.,  forms  a  real  image  of  the  slit  in  the  plane  E  of  the  con- 
denser K.  A  microscope  objective  serves  as  a  condenser  K,  which 
throws  a  reduced  image  of  the  slit  in  the  solution.  The  objective  of 
the  observation  microscope  is  adjusted  over  this  image.  /  and  B  are 
screens,  while  jV  is  a  Nicol's  prism,  which  is  not  necessary  for  ordinary 
work.  The  preparation  to  be  investigated  is  put  into  a  tiny  cuvette 
which  can  be  raised  or  lowered  at  will,  and  is  shown  in  Fig.  3.  The 
cuvette  is  attached  to  the  objective  of  the  observation  microscope  in 
such  a  manner  that  they  can  be  raised  or  lowered  simultaneously  by 

*  H.  Siedentopf  und  R.  Zsigmondy:  Drudes  Annalen  d.  Phys.  (4),  10,  1-39  (1903). 


14 


CHEMISTRY  OF  COLLOIDS 


FIG.  3.    Cuvette. 


means  of  the  adjusting  screws.  This  arrangement  not  only  facili- 
tates rapid  and  easy  adjustment,  but  permits  the  examination  of  a 
number  of  liquids  without  any  further  adjustment  of  the  apparatus. 

It  is  merely  necessary  to  wash  out  the 
cuvette  by  allowing  water  or  other 
cleaning  liquid  to  run  through  it  and 
to  refill  it  with  the  solution  in  question. 
For  full  directions  with  regard  to  the 
necessary  manipulation  the  author  re- 
fers to  his  already  cited  publication, 
and  to  a  pamphlet  issued  by  the  optical 
works  of  Carl  Zeiss  (Sign.  M  229,  Jena, 
1907).  One  learns  best  of  all  from  an 
experimenter  who  has  had  some  experi- 
ence with  the  instrument. 

Among  other  ultramicroscopical  ar- 
rangements that  have  been  constructed 
.is  that  of  Cotton  and  Mouton.  With 
a  simple  piece  of  apparatus  they  have 
carried  out  a  series  of  important  experiments  concerning  the  elec- 
trical migration  of  colloids,  and  the  magneto-optical  properties  of 
hydrosols,  especially  in  the  case  of  colloidal  iron  oxide.  Cotton 
and  Mouton  published  their  investigation  *  in  1906,  fully  describ- 
ing the  apparatus  employed.  The  incident  rays  are  reflected  from 
a  glass  prism  and  form  an  image  of  the  source  of  light  between 
the  cover  glass  and  the  stage  of  the  microscope.  The  rays  do  not 
enter  the  microscope  because  they  are  totally  reflected  by  the  surface 
of  the  cover  glass. 

Likewise  between  the  cover  glass  and  the  stage  of  the  microscope 
bacteria  are  made  visible  by  means  of  an  apparatus  according  to 
Siedentopf.  f  For  similar  purposes  are  employed  the  dark  field  illu- 
mination with  a  mirror  condenser  of  Reichert,|  Ignatowsky,§  the 
cardioid  condenser  of  Siedentopf ,  If  and  the  concentric  condenser  of 
Jentzsch.||  The  three  condensers  last  mentioned  illuminate  the  prep- 
aration to  an  almost  theoretical  degree  of  intensity.  All  these  instru- 
ments have  the  disadvantage,  however,  that  dust  particles,  adsorbed 

*  A.  Cotton  et  H.  Mouton:  Les  ultramicroscopes  et  les  objects  ultramiscopiques. 
Paris  (1906). 

t  H.  Siedentopf:  Zeit.  f.  wiss.  Mikroskopie,  24,  104-108  (1907). 
J  C.  Reichert:  Zeit.  d.  Allg.  Osterr.  Apoth.  Ver.,  Nr.  6  (1908). 
§  W.  v.  Ignatowsky:  Zeit.  f.  wiss.  Mikroskopie,  26,  387-390  (1909). 
If  H.  Siedentopf:  Verb.  d.  Deutsch.    Phys.  Ges.,  12,  6-47  (1910). 
II  F.  Jentzsch:  Verb.  d.  Deutsch.    Phys.  Ges.,  12,  875-991  (1910). 


GENERAL   CONSIDERATIONS  15 

ultramicrons,  etc.,  interfere  with  the  observations,  and  the  prepara- 
tion of  the  material  sufficiently  pure  for  investigation  is,  therefore, 
very  difficult. 

Polarization  by  Small  Particles;  TyndalPs  Phenomenon 

If  an  unpolarized  ray  of  light  falls  on  a  colloidal  solution  a  certain 
amount  of  diffusion  of  the  ray  takes  place,  and  the  reflected  light  ray 
suffers  plane  polarization.  This  phenomenon  is  true  in  general  for 
all  disperse  systems  as  long  as  the  diameter  of  the  particles  is  small 
compared  to  the  length  of  the  light  waves,  and  as  long  as  the  refractive 
index  of  the  disperse  phase  differs  from  that  of  the  disperse  medium. 
Plane  polarization  of  light  by  small  particles  has  long  been  known  as 
the  Tyndall  effect.  We  know  from  the  experiments  of  Tyndall  that 
the  polarization  becomes  more  pronounced  with  the  diminishing  size  of 
the  particles,  and  is  complete  only  when  their  diameter  is  small  com- 
pared with  the  length  of  the  light  waves.  Tyndall  studied  the  phe- 
nomenon with  jets  of  steam  where  the  size  of  the  particles  could  be 
varied  at  will.  Rayleigh  expounded  the  theory  of  this  polarization 
and  found  that  the  intensity  of  the  reflected  ray  was  inversely  pro- 
portional to  the  fourth  power  of  the  wave  length.  He  found  further 
that  the  illumination  from  any  one  particle  was  proportional  to  the 
square  of  the  volume  of  the  particle.  That  is,  the  shorter  waves 
were  more  refracted  than  the  longer,  and  the  illumination  from  the 
particles  became  rapidly  less  as  the  size  decreased.  It  is  a  well- 
known  theory  that  the  blue  of  the  sky  is  due  to  the  refraction  of  light 
rays  by  small  particles.  According  to  Rayleigh  these  particles  may 
be  oxygen  molecules.  This  refraction  was  at  first  held  by  many  to  be 
fluorescence.  The  NicoFs  prism  enables  the  distinction  to  be  made 
very  easily  between  the  two  phenomena. 

The  Determination  of  the  Size  of  Ultramicroscopical  Particles 

Conditions  essential  for  successful  experimentation  with  colloidal 
solutions  will  be  found  comprehensively  described  in  the  already  cited 
publication  of  the  author,  Chapter  6.  The  size  of  the  particle  is 
most  easily  determined  by  counting  the  number  of  them  in  a  given 
volume  of  the  hydrosol  and  applying  the  formula 

A 


S-N' 

where  L  is  the  length  of  the  side  of  one  particle,  A  is  the  mass  of  the 
dispersed  substance  in  the  unit  volume,  N  the  number, of  submicrons 
in  the  same  volume,  and  S  is  the  specific  gravity  of  the  dispersed  sub- 


16  CHEMISTRY   OF  COLLOIDS 

stance.  The  formula  holds  under  the  assumption  that  the  particles  are 
cubical  in  shape,  and  that  they  are  uniformly  distributed.*  A  further 
assumption  is  made  that  the  particles  are  visible  and  exactly  alike. 
If  the  particles  are  not  all  visible  and  the  large  majority  are  submi- 
crons,  the  result  obtained  represents  the  upper  limits.  On  the  other 
hand,  if  the  majority  of  the  particles  are  not  visible  the  result  will  be 
very  wide  of  the  mark.  By  closely  following  the  directions  it  is  pos- 
sible to  arrive  at  a  useful  result  for  the  average  size  of  the  particles. 
Wiegner  f  has  shown  that  by  counting  the  particles  of  suitable  gold 
hydrosols  different  experimenters  at  different  times  with  the  same 
solution  obtain  results  agreeing  with  one  another  within  the  experi- 
mental error. 

The  extraordinary  difference  in  size  of  the  particles  in  ordinary 
suspensions  and  those  in  hydrosols  may  be  realized  from  Table  1, 
which  is  taken  from  the  author's}  publication  alreadyg  cited.  In 
Table  2  the  linear  dimensions  of  ultramicroscopical  gold  particles  are 
compared  to  those  of  the  molecules.  The  figures  require  no  further 
explanation. 

*  H.  Siedentopf  and  R.  Zsigmondy:  Drudes  Annalen  (4)  10,  16-29  (1903). 
t  G.  Wiegner:  Kolloidechem.    Beihefte,  11,  Heft,  6-7,. 213-242  (1911), 
$  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  122.     Jena  (1905). 


Scale  1:10,000 

DIAGRAM  1. 

A.  Corpuscles  in  human  blood.  F.   Globular  bacilli. 

B.  Fragments  of  rice  starch.  /,  g,  h.   Particles  in  colloidal  gold. 

C.  Particles  in  kaolin  suspensions.  i,  k,  L    Particles  that  precipitate  from 
E.   Bacilli  from  a  diseased  spleer.  gold  suspensions.  (17) 


18 


CHEMISTRY  OF  COLLOIDS 


o 

c 


D 


Scale  1:1,000,000 

DIAGRAM  2. 

a.   Hydrogen  molecules. 
6.   Chloroform  molecules. 

c.  Hemoglobin  molecules. 

d,  e,  f,  g.   Particles  in  colloidal  gold. 

h.  Particles  that  precipitate  from  gold  suspensions. 


CHAPTER  II 

CLASSIFICATION 

Change  of  Properties  of  Dispersed  Systems  with  the  Size 
of  the  Particles 

IT  is  a  well-known  fact  that  the  properties  of  rock  fragments  differ 
with  the  size  of  the  fragments.  Geologists  group  the  fragments 
broken  off  from  rocks  into  boulders,  rubble,  pebbles,  gravel,  sand, 
and  dust,  employing  as  a  basis  the  change  of  properties  with  the  size 
of  the  individual  pieces.  For  instance,  sand  and  dust  are  transported 
by  the  wind,  while  the  others  remain  behind.  The  material  of  which 
these  particles  are  composed  makes  little  difference..  Again  sand  will 
pass  through  a  sieve  of  10  mm.  gauge,  and  the  others  will  not.  In 
the  case  of  smaller  subdivisions  also  with  which  we  are  interested 
striking  changes  of  properties  occur  with  changes  in  the  size  of  the 
particles.  Dust  or  kaolin  stirred  in  water  causes  a  turbidity  and  will 
gradually  settle  to  the  bottom.  Hydrosols  in  which  the  particles 
are  less  than  20  MM  remain  quite  clear  and  do  not  settle  at  all.  It 
is  a  well-known  fact  that  between  these  very  fine  subdivisions,  on  the 
one  hand,  and  coarse  suspensions  on  the  other,  there  exist  all  possible 
intermediate  steps.  All  these  subdivisions  may  be  included  in  the 
large  group  of  dispersed  systems.  Nevertheless  the  classification  of 
colloidal  systems  demands  further  grouping  because  the  typical  rep- 
resentatives of  these  groups  have  very  different  properties. 

A  scientific  basis  for  classification  would  be  the  division  into  groups 
where  sudden  changes  of  properties  occur,*  or  where  there  is  a  marked 
change  in  any  one  property.  Fortunately  such  sudden  changes,  or 
breaks,  do  occur.  For  instance  if  one  chooses  the  diameter  or  size  of  the 
particles  as  a  gradually  changing  property,  a  sharp  break  occurs  be- 
tween the  limits  0.1  /*  and  1.0  n,  as  will  be  seen  by  the  following  con- 
siderations. The  resolving  limit  of  the  best  microscope  is  reached  at 
0.2  M,  and  therefore  the  actual  form  of  particles  smaller  than  this 
cannot  be  seen.  In  this  neighborhood  also  lies  the  limit  of  visibility 
with  the  microscope,  using  ordinary  light.  As  a  result  of  this  coinci- 
dence these  two  limits  have  often  been  confused.  The  limit  of  visi- 

*  W.  Ostwald:  Koll.-Zeit.,  1,  297  (1907). 
19 


20  CHEMISTRY  OF  COLLOIDS 

bility  by  means  of  the  microscope  in  transmitted  light  is,  however,  of 
great  importance  because  the  presence  of  tiny  particles,  such  as  occur 
in  colloidal  solutions,  could  not  be  proved  until  the  introduction  of 
ultramicroscopy.  Previous  to  this  microscopic  observers  considered 
such  systems  homogeneous.  Even  chemists  accepted  these  limits, 
and  systems  in  which  the  microscope  no  longer  gave  indications  of 
the  existence  of  individual  particles  were  classified  among  the  homo- 
geneous. 

At  a  very  slightly  lower  limit  than  the  above,  particles  with  even  a 
high  specific  gravity  no  longer  sink  to  the  bottom  in  water,  but  re- 
main suspended,  and  in  constant  vibration.  At  a  slightly  higher  limit 
than  this  lies  the  value  beyond  which  the  Brownian  movement  is  too 
small  to  be  perceived.  Another  limit  of  importance  to  the  chemist 
is  1.0  /*,  because  particles  larger  than  this  may  be  separated  from  the 
medium  by  filter  paper.  If  all  this  is  taken  into  consideration  it  will 
be  seen  that  particles  appreciably  smaller  than  the  light  waves  have 
entirely  different  properties  from  those  that  are  larger,  and  that  a 
number  of  properties  manifest  a  more  or  less  sudden  change  in  the 
region  of  the  resolving  limits  of  the  microscope.  A  brief  summary  of 
these  relations  is  given  in  Table  1. 

TABLE  1 
0.1  MM        IMM        10  MM        100  MM  IM  10  M        100  M  1mm. 

Ultramicroscopic  field  :  Microscopic  field 

.'  Real  form  of  particles  may  be  seen. 

_ __  ' 

Quartz    in    solution    does    not     !  Quartz  solution  forms  a  sediment 
form  a  sediment  ! 

: 

• 

Oil  emulsions   in  water   do  not       :  Oil  emulsions  in  water  separate  into  two 
separate  into  two  layers  ;       layers. 

Particles  pass  through  filter  paper          :  Particles  may  be  filtered  out 

Brownian  Movement 
Very  rapid  Slow  None 

Crystalloidal      :       Hydrosols       : 
solutions         :       Colloidal         : . 

I       Solution    Turbidities          Suspensions 

From  a  brief  review  of  the  systems,  that  up  to  the  present  time  have 
been  considered  hydrosols,  it  will  be  seen  that  few  have  been  ascribed 


CLASSIFICATION  21 

to  this  class  where  the  individual  particles  could  be  seen  under  the 
ordinary  microscope,  because  sedimentation  takes  place  even  when 
the  subdivision  is  still  very  fine.  Especially  is  this  the  case  if  the 
specific  gravity  of  the  disperse  phase  is  appreciably  greater  than  that 
of  the  disperse  medium.  It  is  therefore  possible  without  reserve  to 
set  0.1  /i  as  the  upper  limit  of  size  for  the  particles  in  a  true  hydrosol.* 
The  single  exception  is  that  of  drops  of  oil  in  water.  Without  ques- 
tion the  majority  of  Graham's  colloidal  solutions,  if  carefully  prepared, 
have  a  much  greater  degree  of  homogeneity,  and  contain  much  smaller 
particles. 

From  a  consideration  of  the  fact  that  only  homogeneous  appearing 
mixtures  are  classified  by  chemists  and  physicists  as  solutions,  it 
would  be  well  to  retain  the  term  "  colloidal  solution  "  for  systems 
having  this  apparent  homogeneity,  and,  following  the  suggestion  of 
Quincke,f  to  designate  as  turbidities  all  sols  where  a  turbid  appear- 
ance indicates  inhomogeneity;  that  is  all  systems  that  occupy  a  posi- 
tion intermediate  between  colloidal  solution  and  true  suspensions. 
We  have,  therefore,  the  following  approximate  limits: 

True  suspensions  with  particles  down  to  about  0.1  /*. 

Hydrosols  (colloidal  solutions  and  turbidities)  with  particles  be- 
tween 0.001  M  and  0.1  /*• 

Molecules  of  crystalloids  with  particles  between  0.1  w  and  1.0/ttju. 

It  is  obvious  that  the  division  into  groups  gives  only  an  approxi- 
mate survey  of  the  size  relations  appearing  in  different  sorts  of  finely 
divided  systems,  and  that  a  sharper  subdivision  of  dispersed  systems 
based  on  the  size  of  the  particles  would  be  fraught  with  difficulties, 
because  the  properties  of  finely  divided  matter  are  not  completely 
determined  by  the  size  of  the  particles.  A  number  of  other  factors 
must  be  included  that  depend  upon  the  nature  of  the  dispersed  mate- 
rial, the  disperse  medium,  and  the  presence  of  other  substances. 
From  a  consideration  of  these  circumstances,  without  ascribing  any 
particular  size  to  the  particles,  one  may  assume  that  a  given  system 
belongs  to  the  colloidal,  if  the  effect  of  gravity  is  negligible  compared 
to  the  influence  of  kinetic  and  other  forms  of  energy;  that  is,  when 
the  last-named  factors  determine  the  behavior  of  the  system. 

Suspensions  and  Hydrosols 

The  influence  of  the  size  of  the  particles  is  especially  marked  when 
subdivisions  are  compared,  the  particles  of  which  belong  to  a  differ- 

*  G.  Wiegner:  Zeit.  f.  Kolloidchemie,  Beihefte,  2,  213-242  (1911). 
t  G.  Quincke:  Drudes  Annalen  d.  Phys.  (4),  7,  57-96  (1902). 


22  CHEMISTRY  OF  COLLOIDS 

ent  order  of  magnitude;  that  is,  when  colloids  are  compared  with 
suspensions  that  settle  out  rapidly.  Because  a  great  deal  of  misap- 
prehension exists  with  regard  to  the  behavior  of  true  coarse  suspen- 
sions, the  subject  will  be  taken  up  briefly  at  this  point.* 

Coarse  Suspensions.  —  Frequently  the  behavior  of  irreversible  col- 
loids is  considered  analogous  to  that  of  coarse  suspensions,  although 
the  latter  may  manifest  little  or  none  of  the  properties  of  the  former. 
The  study  of  the  properties  of  such  suspensions,  which  may  be  car- 
ried on  to  advantage  with  potato  starch  (freed  from  smaller  particles), 
where  the  particles  have  a  diameter  lying  between  0.03  and  0.1  mm., 
or  with  quartz  suspensions  with  particles  between  0.1  and  0.2  mm., 
reveals  little  save  sedimentation.  Electrolytes  either  cause  no  per- 
ceptible change  in  the  coagulation,  or  they  retard  it  because  of  the 
increased  density  of  the  medium.  Such  suspensions  are  as  insensi- 
tive to  electrolytes  of  medium  concentration  as  are  proteid  solutions. 

In  the  case  of  finer  suspensions  such  as  wheat  starch  or  quartz 
powder,  the  particles  of  which  have  a  diameter  of  0.001  to  0.005  mm., 
the  effect  of  electrolytes  is  more  marked.  The  particles  flock  together 
to  form  larger  aggregates,  and  these  aggregates  precipitate  more 
rapidly  than  do  the  individual  particles.  The  much  investigated 
turbid  solutions  of  clay  exhibit  this  to  a  marked  degree,  as  shown 
by  the  work  of  Schlosing  f  and  Bodlander.  J  These  solutions  behave 
in  a  manner  very  similar  to  that  of  irreversible  hydrosols  such 
as  colloidal  metals,  which  are  also  very  sensitive  to  the  action  of 
electrolytes. 

The  statement  so  often  met  with  in  the  literature  that  the  precipi- 
tation of  these  finer  suspensions  is  irreversible,  as  in  the  case  of  colloidal 
metals,  is  quite  false.  §  Schlosing  has  demonstrated  that  after  the 
removal  of  the  electrolyte  the  clay  may  be  returned  to  its  original 
suspended  form.  In  this  regard  turbid  clay  solutions  resemble  hydro- 
phile  colloids  (reversible)  much  more  than  they  do  metal  hydrosols, 
and  from  the  properties  of  the  precipitate  should  be  classed  with  the 
former  rather  than  with  the  latter.  The  similarity  between  turbid 
solutions  of  clay  and  metal  hydrosols  exists  almost  exclusively  in  the 
sensitivity  of  both  toward  electrolytes.  This  sensitivity,  however, 
they  have  in  common  with  certain  ionogen  disperse  solutions  such  as 
Congo-red  and  Benzo-purple. 

*  Zsigmondy:  Zur  Erkenntis  der  Kolloide,  11-16. 
t  Ch.  Schlosing:  Compt.  rend.,  70,  1345-1348  (1870). 

J  G.  Bodlander:  Neues  Jahrb.  f.  Min.,  Geol.  usw.,  2,  147-168  (1893);  Nach- 
richten  d.  Kgl.  Ges.  d.  Wiss.  Gottingen.  Math.  phys.  Kl.,  267-276  (1893). 

§  Z.  B.  Wo.  Oswald:  Grundriss  der.  KoUoidchemie,  101-102.     Dresden  (1909). 


CLASSIFICATION  23 

Similarities  between  Colloidal  and  Crystalloidal  Solutions 

Although  the  want  of  homogeneity,  together  with  some  other  prop- 
erties has  been  the  occasion  for  considering  colloidal  solutions  as 
discontinuous,  one  is  not  justified  in  choosing  this  property  as  a 
characteristic  distinction  between  colloidal  and  crystalloidal  solutions. 
For,  on  the  one  hand,  the  optical  inhomogeneity  of  hydrosols  can  be 
made  almost  to  disappear;  while,  on  the  other,  as  Spring,  Lobry  de 
Bruyn,  and  their  fellow  workers  have  shown,  a  large  number  of  crystal- 
loidal solutions  exhibit  the  Tyndall  effect  quite  as  well  as  colloidal 
solutions  do. 

The  existence  of  all  manner  of  gradations  between  the  two  extremes, 
colloidal  and  crystalloidal  solutions,  is  still  further  ground  which  goes 
to  show  that  a  fundamental  distinction  between  these  two  classes  of 
solutions  is  impractical.  Finally,  from  a  geometrical  point  of  view 
such  a  distinction  would  be  possible  only  on  the  assumption  that 
crystalloidal  solutions  are  continuous.  This  hypothesis  is  not  tenable 
because  there  can  no  longer  be  any  doubt  as  to  the  existence  of  mole- 
cules.* 

From  what  has  been  said  it  is  evident  that  the  question,  whether 
hydrosols  are  solutions  or  suspensions,  is  purposeless.  As  a  matter 
of  fact  they  occupy  an  intermediate  position.  They  exhibit  prop- 
erties that  resemble  crystalloidal  solutions  or  suspensions  depending 
upon  the  kind  of  hydrosol  and  upon  the  fineness  of  division.  As 
Nernst  f  has  often  pointed  out,  the  general  behavior  of  most  colloidal 
solutions  would  place  them  with  the  crystalloidal  rather  than  with 
suspensions.  The  fact  that  hydrosols  diffuse,  and  that  they  possess 
osmotic  pressure  would  tend  to  justify  this  point  of  view. 

Electrical  charges  on  the  particles  operate  in  such  a  manner  that 
many  hydrosols  behave  during  electrolysis  precisely  as  electrolytes  with 
large  complex  ions.  Colloidal  stannic  acid,  the  purple  of  Cassius  and 
many  others  may  be  cited  as  examples.  The  reactions  of  hydrosols 
are  often  determined  by  the  electrical  charge  and  are  generally  char- 
acteristic of  the  sign.  They  are  dependent  upon  the  nature  of  the 
disperse  phase  just  as  in  the  case  of  crystalloidal  solutions.  The  par- 
ticles of  reversible  and  also  the  irreversible  hydrosols  behave  similarly 
to  molecules  and  ions  in  that  they  are  adsorbed  by  various  substances. 

Although  the  occurrence  of  sudden  breaks  in  the  properties  of  the 
solution  marks  the  noticeable  difference  between  a  typical  colloidal 
and  crystalloidal  solution,  nevertheless  these  same  sudden  breaks  are 

*  W.  Mechlenburg:  Die  experimentelle  Grundlegung  der  Atomistik.    Jena  (1910). 
t  W.  Nernst:  Theoretische  Chemie,  5.    Aufl.,  415. 


24  CHEMISTRY  OF  COLLOIDS 

also  met  with  in  systems  having  properties  intermediate  between  the 
two  main  classes.  For  instance  breaks  occur  in  the  case  of  suspensions 
in  the  neighborhood  of  the  resolving  power  of  the  microscope,  while 
this  same  phenomenon  is  manifested  in  crystalloidal  solutions  where 
the  particles  have  the  dimensions  of  about  1  /x/i. 

Behavior  of  Hydrosols  on  Evaporation 

As  has  already  been  pointed  out  on  page  10,  colloids  can  be  divided 
into  the  two  classes,  reversible  and  irreversible,  depending  upon 
whether  or  not  they  leave  a  soluble  residue  on  evaporation.  The 
irreversible  can  be  still  further  divided  into  two  groups.  1.  To  the 
first  class  belong  those  that  coagulate  in  dilute  solution,  and  precipi- 
tate in  the  form  of  a  powder  rather  than  a  jelly.  Examples  of  these 
are  the  colloidal  metals  in  a  pure  state  (colloidal  metals  that  are  not 
rendered  impure  by  the  presence  of  any  other  colloid) .  2.  The  second 
class  consists  of  those  that  may  be  considerably  concentrated  before 
coagulation  sets  in  and  whose  precipitates  are  decidedly  j-elly-like,  such 
as  colloidal  silicic  acid,  stannic  acid,  clay,  iron  oxide  or  hydroxide. 

Colloids  of  the  first  group,  which  may  be  considered  as  completely 
irreversible,  are  coagulated  by  all  of  the  well-known  methods,  such  as 
evaporation,  addition  of  an  electrolyte,  freezing,  etc.  The  coagula- 
tion is  so  thorough  that  the  colloid  cannot  be  brought  back  into  its 
original  state  either  by  raising  the  temperature,  by  dilution  with 
water,  removal  of  the  electrolyte,  or  by  peptisation.  In  order  to  make 
a  hydrosol  out  of  the  residue  electrical  or  chemical  energy  is  im- 
perative. 

With  the  irreversible  colloids  of  the  second  class,  on  the  other  hand, 
it  is  possible  to  reform  the  hydrosol  by  the  addition  of  a  small  amount 
of  a  suitable  reagent  provided  that  the  residue  has  not  been  too 
thoroughly  dehydrated.  A  too  complete  dehydration  causes  a  con- 
tinuous change  to  take  place  which  may  proceed  so  far  that  the  dry 
residue  will  no  longer  undergo  peptisation.  (Example,  colloidal  stan- 
nic acid.) 

Lying  in  a  position  intermediate  between  hydrosols  of  pure  metals 
and  irreversible  oxides  will  be  found  the  majority  of  well-dialyzed  sul- 
fide  hydrosols.  On  evaporation  these  sometimes  give  jelly-like  bodies 
and  sometimes  precipitate  in  a  powder  form. 

Typical  reversible  colloids,  after  sufficient  swelling  has  occurred, 
dissolve  in  a  solvent  to  give  a  homogeneous  appearing  hydrosol.  Gum 
arabic,  albumin,  hemoglobin,  and  PaaPs  colloidal  palladium  are  good 
examples.  Substances,  such  as  glue,  soluble  starch,  gelatin,  agar- 
agar,  and  many  others,  constitute  a  special  class  of  reversible  colloids 


CLASSIFICATION  25 

that  have  the  peculiarity  of  hardening  into  the  form  of  a  gel  on  cooling. 
The  dried  residue  of  these  bodies  will  swell  up  to  a  certain  degree  in 
water  at  ordinary  temperatures,  but  the  raising  of  the  temperature 
causes  complete  and  easy  dissolution.  In  fact  the  frequent  occurrences 
of  bodies  having  this  peculiarity  led  Graham  to  choose  glue  as  the 
typical  representative  of  colloids. 

Another  class  of  reversible  colloids  deserves  mention.  They  may 
be  called  half-  or  semi-colloids  because  they  diffuse  in  water  very 
slowly  through  membranes.  They  also  exhibit  a  certain  amount  of 
osmotic  pressure,  and  lower  the  boiling  point  appreciably.  These 
properties  would  place  them  in  a  position  midway  between  colloids 
and  crystalloids.  A  large  number  of  substances  belong  to  this  class; 
on  the  one  hand  decomposition  products  of  true  colloids,  such  as  dex- 
trin and  peptones;  on  the  other,  aqueous  solutions  of  salts  of  organic 
substances  possessing  high  molecular  weights,  such  as  stearic  and 
oleic  acids,  and  the  salts  of  acid  and  basic  dyestuffs.  Salts  of  dye- 
stuffs  are  perhaps  to  be  regarded  as  mixtures  of  colloids  and  crystal- 
loids. Soap  in  water  doubtless  belongs  to  this  class.* 

The  Behavior  of  the  Specific  Groups  Toward  Electrolytes 

No  very  general  statement  can  be  made  with  regard  to  the  effect  of 
electrolytes  on  hydrosols.  It  is  possible,  however,  to  enunciate  a 
large  number  of  laws,  which  have  many  exceptions  when  specific 
groups  are  considered. 

A.  Irreversible  Colloids.  —  Some  irreversible  colloids  are  sensitive 
to  electrolytes,  while  others  remain  unaffected.  Some  degree  of  regu- 
larity of  behavior  is  manifested  only  by  colloidal  metals  (free  from 
contamination  with  other  colloids),  salts,  and  sulfides.  Here  the  law 
obtains  with  a  fair  amount  of  certainty,  that  electrolytes  cause  pre- 
cipitation. In  general  very  small  concentrations  of  salts,  bases,  and 
acids  suffice  to  coagulate  the  colloid  in  question.  The  precipitate  is 
not  soluble  in  water.  Nonelectrolytes,  on  the  contrary,  usually  do 
not  cause  coagulation.  The  sensitiveness  may  be  completely  de- 
stroyed by  the  addition  of  often  very  small  quantities  of  protective 
colloids.  Even  traces  of  these  last-named  substances  may  influence 
the  reaction  to  a  considerable  degree. 

Colloidal  oxides  in  part  obey  the  above  law;  in  part  show  a  specific 
behavior  for  each  individual  case.  For  instance  colloidal  stannic  and 
ferric  oxides  are  immediately  precipitated  by  most  electrolytes;  while 
silicic  acid,  zirconium  and  thorium  oxides,  and  many  others  show 

*  McBain:  Jour.  Chem.  Soc.,  101,  2042  (1912). 


26  CHEMISTRY  OF   COLLOIDS 

specific  reactions.  Whether  the  product  of  the  precipitation  is  sol- 
uble in  water  or  not  depends  upon  the  nature  of  both  the  colloid  and 
the  electrolyte. 

B.  Reversible  Colloids.  The  reversible  hydrosols  are  in  general 
not  very  sensitive  toward  the  salts  of  the  alkali  metals.  Large  quan- 
tities of  these  salts  are  usually  necessary  to  produce  precipitation. 
The  reaction  is  mostly  reversible.  Toward  the  salts  of  the  heavy 
metals  these  colloids  act  in  a  manner  similar  to  that  of  the  irreversible, 
inasmuch  as  often  exceedingly  small  quantities  are  sufficient  for  pre- 
cipitation. In  all  these  cases  specific  effects  are  manifested  that 
render  a  further  subdivision  into  classes  extremely  difficult. 

Contrary  to  their  usual  behavior,  which  would  place  them  in  the 
class  with  the  electrolytes,  solutions  of  dyestuffs  frequently  exhibit 
a  sensitiveness  toward  electrolytes  very  similar  to  that  of  metal  col- 
loids. The  precipitates  caused  by  the 'salts  of  the  alkali  metals  can 
be  easily  returned  to  the  original  form  just  as  in  the  case  of  suspen- 
sions of  clay. 

In  view  of  the  varied  behavior  of  colloids  toward  electrolytes  it 
seems  scarcely  possible  to  classify  them  according  to  this  property. 
By  studying  the  reactions  of  colloids  collectively  toward  specific 
reagents  it  might  be  possible  to  arrive  at  a  classification  that  would 
resemble  the  division  into  groups  in  analytical  chemistry.  Such  a 
classification,  however,  would  not  give  a  sharp  line  of  distinction  be- 
tween the  various  groups,  because  one  and  the  same  colloid  may  ex- 
hibit different  reactions,  depending  upon  its  age  and  the  method  of 
preparation. 

Other  Classifications 

The  differences  already  mentioned  between  substances  of  the  type 
of  colloidal  gold,  on  the  one  hand,  and  egg  albumin  on  the  other,  have 
long  been  recognized.  As  a  consequence  many  attempts  have  been 
made  to  classify  colloids  with  these  differences  as  a  basis.  Noyes 
proposed  to  distinguish  two  groups,  colloidal  suspensions  and  colloidal 
solutions.  The  former  he  characterized  as  nonviscous,  nongelati- 
nous,  and  easily  precipitated  by  electrolytes.  The  latter  as  viscous, 
gelatinous,  and  difficultly  precipitable  by  electrolytes.  Such  a  classi- 
fication is  not  suitable  because  it  takes  too  many  characteristics  into 
consideration.  There  are  many  colloids  that  would  not  come  under 
these  headings.  For  instance  Paal's  colloidal  silver  is  nonviscous, 
and  precipitated  by  electrolytes  with  great  difficulty.  Such  a  point 
of  view  would  often  leave  one  in  grave  doubt  as  to  which  class  col- 
loidal oxides,  sulfides,  and  salts  belonged.  The  properties  chosen  for 


CLASSIFICATION  27 

this  classification  would  serve  to  characterize  a  few  extreme  members 
differing  widely,  but  would  not  be  suitable  for  the  large  majority  of 
colloidal  substances. 

Several  other  names  have  been  suggested  from  time  to  time  instead 
of  colloidal  suspensions.  Perrin  advocates  hydrophobe  colloids; 
Freundlich,  lyophobe  colloids;  Hober  and  Wo.  Ostwald,  suspension 
colloids;  von  Weimarn,  suspensoids,  and  so  on.  Instead  of  Noyes's 
colloidal  solutions,  these  authors  have  suggested  hydrophiles,  lyo- 
philes,  emulsion  colloids,  and  emulsoids,  respectively.  The  author  can- 
not agree  with  a  classification  on  the  basis  chosen.  The  number  of 
characteristics  has  been  further  increased  by  certain  authors.  The 
designations,  suspensions,  and  emulsions  give  ideas  about  the  state  of 
aggregation  of  the  finely  divided  body  that  often  do  not  correspond 
to  reality.  From  these  considerations  colloidal  mercury  would  be- 
long to  emulsion  colloids;  yet  its  properties  determine  its  place  to 
be  among  the  suspension  colloids.  Regarded  as  a  finely  divided  solid 
substance  colloidal  gold,  protected  by  a  small  amount  of  gelatin, 
would  fall  into  the  class  with  other  suspension  colloids.  Its  property 
of  withstanding  the  action  of  electrolytes)  and  also  the  fact  that  it 
will  gelatinize  at  higher  concentrations  makes  it  conform  very  well  to 
the  definition  of  emulsion  colloids. 

Disperse  Systems 

Hydrosols  and  hydrogels,  the  discussion  of  which  will  occupy  the 
greater  part  of  this  book,  fall  into  the  class  of  finely  divided  substances 
or  disperse  systems.  The  divided  substance  Wo.  Ostwald  *  designates  as 
the  "disperse  phase,"  the  medium  in  which  the  particles  are  suspended 
as  the  "disperse  medium."  Some  English  authors  had  previously  used 
the  terms  "internal  phase,"  and  "external  phase,"  for  disperse  phase 
and  disperse  medium  respectively.  The  French  have  used,  "granules 
colloidaux,"  and  "milieu  exterieur." 

Wo.  Ostwald,  following  the  classification  of  the  author,  has  divided 
disperse  systems  into  three  subdivisions  according  to  the  size  of  the 
particles. 

1.  Coarse  dispersoids;  suspensions  and  emulsions. 

2.  Disperse  systems  that  lie  between  suspensions  and  crystalloid 
solutions;  colloidal  solutions. 

3.  Molecular  and  ionic  disperse  systems;    crystalloidal  solutions, 
both  electrolytes  and  nonelectrolytes. 

*  Wo.  Ostwald:  Koll.-Zeit.,  1,  291-300,  331-341  (1907);  Grundriss  der  Kolloid- 
ehemie,  83.  Dresden  (1909). 


28  CHEMISTRY   OF  COLLOIDS 

He  further  divides  disperse  systems  into  9  classes  according  to  the 
state  of  aggregation  of  the  disperse  phase  and  disperse  medium.  Some 
of  these  classes  play  a  very  important  role. 

EXAMPLES 

1.  S  +  S  (colored  sodium  chloride,  ruby  glass). 

2.  S  -f-  L  (minerals  with  enclosed  liquid) . 

3.  S  +  G  (minerals  with  enclosed  gas). 

4.  L  +  S  (suspensions  and  hydrosols  with  solid  particles). 

5.  L  +  L  (emulsions  and  colloidal  solutions  with  liquid  particles) . 

6.  L  +  G  (foam). 

7.  G  +  S  (smoke,  cosmic  dust). 

8.  G  +  L  (mist). 

9.  G  +  G. 

S  =  solid;  L  =  liquid;  G  =  gas. 

This  classification  gives  a  comprehensive  view  of  a  large  number  of 
disperse  systems  and  permits  an  orderly  arrangement  into  groups  just 
as  soon  as  the  state  of  aggregation  of  the  disperse  phase  is  known.  No 
one  will  doubt  that  a  suspension  of  kaolin  belongs  to  the  class  L  +  S; 
an  oil  emulsion  to  L  +  L;  a  low  hanging  rain  cloud  in  the  tropics  to 
G  +  L.  Difficulties  present  themselves,  however,  just  as  soon  as  the 
determination  of  the  state  of  aggregation  of  the  disperse  phase  becomes 
doubtful  or  impossible.  Classification  according  to  this  plan  becomes 
merely  arbitrary.  We  must  therefore  discard  in  this  book  any  classi- 
fication that  involves  the  form  of  the  disperse  phase. 

It  will  be  seen  that  the  effect  of  Ostwald's  classification  of  colloidal 
chemistry  is  to  stimulate  research  for  methods  of  determining  the 
state  of  aggregation  of  the  finely  divided  substances.  The  principles 
of  this  classification  may  be  employed,  however,  as  far  as  they  are 
applicable.  This  is  the  case  if  we  consider  only  three  instead  of  Ost- 
wald's  nine  classes. 

Disperse  systems  with: 

a.  A  gas  as  disperse  medium. 

b.  A  liquid  as  disperse  medium. 

c.  A  solid  as  disperse  medium. 

Just  as  soon  as  the  state  of  aggregation  of  the  disperse  phase  is 
known  Ostwald's  classification  may  be  further  employed  without 
hesitation. 

A  few  examples  of  disperse  systems  with  a  gas  or  a  solid  disperse 
medium  will  now  be  taken  up. 


CLASSIFICATION  29 

Disperse  Systems  with  Gas  as  Disperse  Medium 

Disperse  systems  with  air  as  the  disperse  medium  and  water  as  the  dis- 
perse phase,  such  as  mist  or  rain,  are  of  great  importance  for  meteorology 
and  agriculture;  while  finer  subdivisions  of  water  are  involved  in  the 
blue  color  of  the  heavens. 

As  disperse  systems  with  solid  disperse  phase  may  be  cited  snow 
clouds,  smoke,  cosmic  dust,  and  volcanic  ashes.  The  latter  are  often 
so  fine  that  they  may  be  carried  hundreds  of  miles  by  the  wind,  and  be- 
come the  cause  of  striking  and  beautiful  color  displays  in  the  heavens. 
Not  infrequently  has  it  destroyed  large  tracts  of  land,  and  cities  with 
their  inhabitants.  Herculaneum  and  Pompeii  fell  victims  to  a  rain  of 
ashes;  while  St.  Pierre  was  destroyed  by  a  "nuee  ardente,"  a  hot  heavy 
cloud  composed  of  air,  steam,  small  stones,  sand,  and  dust.  The  dust 
was  in  such  a  state  of  fine  division  that  it  fell  to  the  ground  during  the 
descent  of  the  cloud.* 

In  other  cases  on  record  where  the  concentration  of  the  dust  was 
not  so  high  the  cloud  rose  and  spread  itself  out  in  thin  layers,  form- 
ing strange  and  wonderful  forms.  Without  doubt  the  dust  particles 
are  charged  electrolytically  and  for  this  reason  are  prevented  from 
uniting.  If  during  thunder  storms  the  particles  become  discharged 
they  unite  together  and  fall  in  showers  of  ashes.  The  history  of  the 
volcanic  eruptions  of  Mount  Vesuvius  affords  many  examples  of  this 
phenomenon. 

The  origin  of  this  dust  and  the  clouds  of  ashes  may  very  well  be  ex- 
plained on  the  grounds  that  water  vapor  in  the  interior  of  the  volcano 
at  a  high  temperature  and  under  an  enormous  pressure  is  dissolved  in 
the  silicates,  or  forms  a  sol  with  them.  When  the  pressure  is  suddenly 
released  explosions  occur  owing  to  the  expansion  of  the  enclosed  water. 
The  silicates  would  thus  be  blown  to  pieces,  a  large  portion  of  which 
would  become  dust  and  form  a  disperse  phase  in  water  vapor  as  a  medium. 
This  dust  mixed  with  some  moisture  might  then  be  precipitated  as 
"nuee  ardente,"  or  might  fall  in  a  comparatively  dry  state  as  ashes. 
Barus  f  has  shown  how  such  solutions  of  colloidal  silicates  may  originate. 
Soft  glass,  such  as  is  used  for  glass  tubing,  swells  in  water  at  high  tem- 
perature and  pressure  similar  to  the  manner  in  which  gelatin  does 
under  ordinary  conditions.  If  now  the  temperature  is  sufficiently 
raised  the  jelly-like  mass  melts  to  a  liquid.  On  cooling  under  pressure 
the  liquid  hardens  and  becomes  a  brittle  amorphous  glass,  "waterglass." 
On  heating  under  atmospheric  pressure  this  mass  swells  and  resembles 

*  Lecroix:  La  montagne  Pelee  et  ses  Eruptions.     Paris  (1904),  Masson  et  Cie. 
t  C.  Barus:  Amer.  Journ.  of  Sc.  (4),  9,  161-175  (1900). 


30  CHEMISTRY  OF  COLLOIDS 

foam.  These  are  the  conditions  necessary  for  the  origin  of  "nuee 
ardente"  as  well  as  the  masses  of  pumice  stone  that  are  often  thrown 
out  from  volcanoes. 

Disperse  Systems  with  a  Liquid  as  Disperse  Medium 

In  the  category  of  disperse  systems  with  a  liquid  medium  colloidal 
solutions  must  be  included,  especially  hydrosols  and  hydrogels  with 
water  as  the  disperse  medium.  A  great  many  other  liquids  may  serve 
as  a  disperse  medium.  In  point  of  fact  nonaqueous  colloidal  solutions 
have  assumed  a  very  great  importance  in  the  industries.  Solutions  of 
gums,  rubber,  and  guttapercha  are  examples  of  these.  Their  thorough 
investigation  will  doubtless  form  a  not  unimportant  contribution  to 
the  chemistry  of  colloids.  Considerable  work  *  has  already  been  done 
along  these  lines  that,  owing  to  lack  of  space,  cannot  be  taken  up  at  this 
point. 

Disperse  Systems  with  a  Solid  as  Disperse  Medium 

Among  the  systems  with  a  solid  for  the  disperse  medium  several  are 
of  importance  and  have  been  investigated.  Only  two  of  them,  ruby 
glass  and  hardened  pyrosols,  will  be  taken  up  here.  Many  colored 
minerals  also  belong  to  this  class.  A  thorough  investigation  with  the 
ultramicroscope  would  perchance  reveal  how  much  more  frequent  dis- 
continuity is  than  scientists  have  been  inclined  to  believe.  Even 
crystals  that  have  always  been  regarded  as  homogeneous  turn  out  very 
often  to  be  discontinuous. 

Ruby  glass  is  obtained  by  melting  suitable  glass  with  gold,  silver, 
copper,  or  with  compounds  of  these  metals.  The  most  thoroughly 
studied  is  gold  glass,  which  has  played  an  important  part  in  the  devel- 
opment of  ultramicroscopical  methods. t  Gold  ruby  glass  can  be  made 
by  melting  lead  or  barite  glass  and  adding  a  very  small  amount  of  gold 
chloride.  If  the  glass  is  quickly  cooled  it  is  usually  colorless,  but  be- 
comes colored  if  slowly  reheated.  The  colorless  glass  must  be  consid- 
ered as  a  solution  of  metallic  gold  in  the  silicate  mass,  and  not  as  a 
chemical  combination  of  gold.  By  slow  cooling,  or  better  still,  by 
careful  reheating  numberless  submicrons  of  metallic  gold  are  formed  in 
the  mass,  and  these  give  rise  to  the  color.  Their  diameter  corresponds 
to  that  of  the  particles  in  a  colloidal  gold  solution.  Both  submicrons 
and  amicrons  are  present;  the  smallest  visible  with  the  ultramicroscope 

*  Koll.-Zeit,,  1,  33-65  (1906-7)  von  C.  O.  Weber;  165  von  R.  Ditmar;  4,  74 
(1909)  von  D.  Spencer;  6,  31  (1909)  von  H.  W.  Woudstra;  6,  136  (1910)  von  Wo. 
Ostwald,  202  von  F.  Hinrichsen  u.  E.  Kindscher,  281  von  B.  Bysow  u.  a.  m. 

t  H.  Siedentopf  und  R.  Zsigmondy:  Drudes  Annalen  d.  Phys.  (4),  10,  1-39  (1903). 


CLASSIFICATION  31 

have  a  mass  of  about  10~15  mg.  A  theory  for  the  formation  of  ruby  glass 
has  been  published  by  the  author  employing  as  a  basis  the  fundamental 
principles  of  glass  formation  by  Tammann.* 

Colored  Sodium  Chloride.  —  Siedentopff  has  shown  that  colored 
sodium  chloride,  which  can  be  obtained  by  heating  in  vacuum  in  the 
presence  of  sodium  vapor,  is  filled  with  ultramicrons.  These  have  the 
appearance  and  show  all  the  properties  of  a  finely  divided  metal.  Pre- 
cisely as  in  the  case  of  ruby  glass,  the  color  of  sodium  chloride  changes 
with  the  temperature.  These  color  variations  are  attributed  to  changes 
in  the  metallic  submicrons.  Concerning  the  polarization  of  submi- 
crons,  see  Chapter  V. 

Pyrosols.  —  R.  Lorenz  J  defines  pyrosols  as  colloidal  solutions  of 
metals  and  other  substances  in  a  red  hot  liquid  medium.  Colloidal 
metal  solutions  in  fused  salts  have  been  correctly  recognized  and  closely 
studied.  They  play  an  important  part  in  the  electrolysis  of  fused 
salts,  and  their  presence  explains  some  of  the  anomalous  yields  obtained 
during  the  decomposition  of  fusions  by  the  electric  current. 

The  easiest  method  of  preparing  pyrosols  is  by  dissolution  of  the 
metal  in  the  fused  salt,  e.g.,  zinc  in  zinc  chloride,  cadmium  in  cad- 
mium chloride.  During  the  reaction  at  suitable  temperatures  some- 
times slight  explosions  take  place,  whereby  the  metals  give  off  colored 
vapor.  These  phenomena  have  nothing  whatever  to  do  with  the  forma- 
tion of  subhalides,  as  has  been  shown  by  Lorenz.  §  The  formation  of 
pyrosols  is  connected  with  the  increase  of  the  vapor  pressure  with  the 
temperature.  When  the  vapor  tension  of  the  metal  is  appreciable, 
the  formation  of  the  color  is  more  pronounced.  On  cooling  some 
pyrosols  are  permeated  with  lustrous  particles.  The  intensive  color 
of  the  mass,  and  the  fact  that  the  color  may  be  destroyed  by  salting 
out  the  pyrosol  with  sodium  or  potassium  chloride,  are  evidences  that 
the  metal  is  present  in  a  colloidal  state.  The  homogeneous  appear- 
ance bespeaks  the  very  fine  state  of  division.  Lorenz  has  thrown  light 
on  the  relation  of  pyrosols  to  ruby  glass. 

Classification  Employed  in  this  Book 

Various  considerations  have  actuated  the  author  to  choose  for  the 
special  part  of  this  work,  a  classification  differing  from  any  yet  sug- 
gested. For  the  sake  of  comprehensiveness  the  special  colloids  to  be 

*  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  128-135. 
t  N.  Siedentopf:  Verb.  d.  Deutsch.    Phys.  Ges.,  7,  268-286  (1905). 
J  R.  Lorenz:  Elektrolyse  geschmolzener  Salze.  Halle  (1905).    II.  Teil,  40.  —van 
Bemmelen-Gedenkboek,  395-398  (1910). 
§  R.  Lorenz:  1.  c. 


32  CHEMISTRY  OF  COLLOIDS 

taken  up  are  placed  in  the  following  groups.  The  discussion  is  confined 
almost  exclusively  to  hydrosols  and  hydrogels  because  of  their  pre- 
eminent importance,  while  organosols  and  pyrosols  have  been  relegated 
to  a  subordinate  place. 

I.  INORGANIC  COLLOIDS 

A.  Metals: 

1.  Pure  metal  hydrosols. 

2.  Metal  colloids  with  protective  colloids. 

B.  Other  elements  (S,  Se,  etc.). 

C.  Oxides. 

D.  Sulfides. 

E.  Salts. 

II.  ORGANIC  COLLOIDS 

A.  Organic  salts. 

1.  Soaps. 

2.  Dyestuffs. 

B.  Protein  bodies. 

As  examples  especial  mention  is  made  of  albumin,  globin,  gelatin, 
hemoglobin,  casein.  To  these  might  be  added  carbohydrates,  such  as 
cellulose,  starch,  dextrin,  etc.;  also  colloids  that  are  soluble  in  an 
organic  medium,  such  as  resins,  rubber,  etc.  To  the  inorganic  colloids 
ruby  glass  and  pyrosols  must  be  added. 

Because  it  has  been  possible,  following  out  the  plan  of  this  book,  to 
choose  specific  colloids  so  typical  of  the  class  to  which  they  belong  that 
they  suffice  to  bring  out  all  the  important  chemical  relations  involved 
in  colloidal  chemistry,  the  other  members  have  been  largely  disregarded. 
The  author  has  made  it  a  fundamental  principle  to  take  up  only  those 
colloids  the  properties  of  which  are  known  to  him  through  his  own 
investigations. 


CHAPTER  III 

PROPERTIES   OF   COLLOIDS 

Diffusion  and  Osmotic  Pressure 

DIRECT  measurements  of  diffusion  of  hydrosols  were  made  by  Gra- 
ham *  and  later  by  Arrhenius  f  and  Herzog.  J  The  diffusion  constants, 

cm 
Z),  expressed  as  -  ~  X  105,  are  generally  very  small.     For  instance,  D 

Sx3C« 

for  egg  albumin  is  0.052.     This  would  correspond  to  a  molecular  weight 
of  20,000. 

The  determination  of  the  lowering  of  the  freezing  point  or  the  rais- 
ing of  the  boiling  point  gives  results  with  colloids  much  too  inaccurate 
to  be  of  any  use.  The  measurement  of  the  osmotic  pressure  has  not 
only  the  advantage  of  greater  delicacy,  but  by  this  method  the  harmful 
influences  of  crystalloidal  impurities  may  be  greatly  reduced  if  not 
completely  eliminated.  The  results  obtained  with  suitable  membranes 
by  Pfeffer,§  Rodewald  and  Kattein,1f  Duclaux,||  Moore  and  Roaf,** 
Lillie,ft  Biltz,it  and  others  will  be  taken  up  later.  Many  colloids,  of 
which  colloidal  iron  oxide  is  an  example,  give  a  very  considerable  rise  in 
the  column,  while  others  cause  little  change.  In  most  cases  the  osmotic 
pressure  changes  with  the  concentration,  but  the  law  governing  this 
change  has  not  yet  been  discovered.  The  dependence  of  the  osmotic 
pressure  upon  the  temperature  has  been  remarked  by  Duclaux  and 
also  by  the  author  in  the  case  of  colloidal  iron  oxide.  It  is  hoped 
that  further  work  in  this  field  will  add  to  our  knowledge  of  specific 
hydrosols. 

*  Th.  Graham:  Phil.  Transact.,  183  (1861);  Liebigs  Annalen,  121,  13  (1862). 
t  Sv.  Arrhenius:  Immunochemie,  17  (1907). 
j  R.  O.  Herzog;  Zeit.  f.  Elektrochemie,  13,  533-9  (1907). 
§  W.  Pfeffer:  Osmotische  Untersuchungen.     Leipzig  (1877). 
If  H.  Rodewald  und  A.  Kattein:  Zeit.  f.  Phys.  Chemie,  33,  579-592  (1900). 
II  J.  Duclaux:  Compt.  rend.,  140,  1468,  1544-1547  (1905);  Joura.  de  chim.  phys., 
6,  29-56  (1907). 

**  B.  Moore  and  H.  E.  Roaf:  Biochemical  Journ.,  2,  34  (1906);  3,  55  (1907). 
ft  R.  S.  Lillie:  Amer.  Journ.  of  Physiol.,  20,  127-169  (1907). 
it  W.  Biltz  und  A.  v.  Vegesack:  Zeit.  f.  phys.  Chemie,  68,  357-382  (1909);   73, 
481-512  (1910). 

33 


34 


CHEMISTRY  OF  COLLOIDS 


Dialysis  and  Ultrafiltration 

The  property  of  plant,  animal  or  artificial  membranes  to  prevent 
colloids  from  passing  through  them  has  made  two  very  important 
methods  of  separation  possible.  The  first  one,  dialysis,  which  Graham 
used  with  such  notable  results,  has  already  been  spoken  of.  A  few 
other  methods  of  employing  this  process  are  described  below.  The 
second,  ultrafiltration,  has  in  the  last  few  years  come  into  general  use 
in  colloidal  chemistry.  It  has  already  yielded  very  important  results 
and  with  further  investigation  should  prove  almost  indispensable  in 
this  field. 

Apparatus  for  Dialysis.  Beside  Graham's  dialyzer,  already  de- 
scribed on  page  7,  several  others  are  in  use.  Generally  a  membrane  is 
stretched  across  an  open  vessel  and  the  whole  partially  submerged  in 
a  larger  vessel  filled  with  distilled  water  as  in  Fig.  4.  If  continuous 

dialysis  is  desired  water  is  conducted 
into  the  outer  vessel  and  out  again 
by  means  of  suitably  bent  tubing. 
If  no  dialyzer  is  at  hand  the  process 
may  be  carried  out  by  means  of  a 
bag  of  parchment  membrane  pressed 
into  the  water  and  filled  with  the 
FIG.  4.  Dialyzer.  solution  to  be  dialyzed.  Kuehne's 

dialyzer,  Fig.  5,  consists  of  a  high  glass  cylinder  which  serves  to  re- 
ceive the  membrane.  This  latter  is  a  U-shaped  tube 
made  of  parchment  membrane.  The  funnel  and  the 
side  tube  serve  to  keep  the  water  renewed.  Unfor- 
tunately the  parchment  tubes  on  the  market  are  so 
thin  that  the  employment  of  this  sort  of  dialyzer  is 
rendered  difficult.  Finally  Schleicher  and  Schull 
have  used  a  dialyzer  that  consists  of  a  parchment 
tube  closed  at  one  end.  A  dialyzer  has  been  em- 
ployed in  the  sugar  industry  for  a  long  time  for  the 
filtration  of  molasses.  Jordis  *  has  constructed  a 
similar  apparatus  on  a  small  scale. 

A  dialyzer  that  permits  a  very  rapid  purification 
of  the  colloid  has  been  described  by  Zsigmondy  and 
Heyer,f  of  which  Fig.  6  is  a  diagram.  A  plate  of 
hard  rubber  shaped  like  a  crystallizing  dish,  having 
a  diameter  of  about  25  to  40  cm.  and  sides  3  to  4  mm. 
*  E.  Jordis:  Zeit.  f.  Elektrochemie,  8,  677-678  (1902). 


FIG.    5.       Dialyzer 
according  to  Kiihne. 


f  R.  Zsigmondy  und  R.  Heyer:  Zeit.  f.  anorg.  Chemie,  68,  169-187  (1910);  Koll.- 
Zeit.,  8,  123-126  (1911). 


PROPERTIES  OF  COLLOIDS 


35 


FIG.  6. 


Cross  section  of  Stern's 
B,  ring  with 


high,  is  bored  through  in  the  center.  Eight  small  strips,  3  to  4  mm.  high, 
are  placed  radially  on  the  plate  and  reach  to  within  1  cm.  of  the  outer  edge. 
These  serve  to  direct  the  stream  of  water 
from  the  center  outwards.  A  ring  of 
hard  rubber  (B)  about  40  mm.  high  sits 
exactly  upon  the  edges  of  plate.  The 
membrane  is  stretched  across  the  under 
edge  of  the  ring.  By  means  of  this  ar- 
rangement a  layer  of  water  3  to  4  mm. 
deep  is  enclosed  between  the  membrane 
and  the  plate.  The  water  is  continu- 
ously renewed  through  the  hole  in  the 
center  of  the  plate,  directed  toward  the 
outer  edge  by  the  strips  and  flows  off 
through  small  holes  in  the  upper  edge  of 
the  plate.  It  is  rather  difficult  to  place 
the  plate  quite  level  and  the  water  is 

therefore  inclined  to  flow  faster  through 

.  ,,      ,    ,        ,,-        ,T  ,       ,.,  dialyzer.    A,  plate. 

some  of  the  holes  than  through  others.  membrane. 

To  avoid  this  strips  of  filter  paper  are 

clamped  between  the  edges  of  the  plate  and  the  bottom  of  the 
ring.  These  strips  operate  as  syphons  for  the  water.  Ordinary  parch- 
ment paper  is  used  as  a  membrane  and  has  proved  itself  quite  dura- 
ble. It  has  the  disadvantage  of  offering  considerable  resistance  to 
the  passage  of  crystalloids.  Of  late  the  so-called  fish  bladder  mem- 
branes have  been  found  very  suitable  for  rapid  dialysis.  They  are  to 
be  had  of  fairly  good  quality  on  the  market.  Recently  collodion 
membranes  which  can  be  made  in  any  desired  form  have  been  em- 
ployed for  the  dialysis  of  colloids  with  excellent  results.  The  rapidity 
with  which  they  allow  the  process  of  dialysis  to  take  place  can  be  shown 
in  a  lecture  experiment.  Parchment  paper  and  a  membrane  of  collo- 
dion are  each  allowed  to  hang  in  the  form  of  a  sack  in  a  vessel  filled 
with  water.  A  fairly  concentrated  solution  of  fluorescein  is  poured  on 
each  membrane.  In  a  short  time  the  fluorescein  will  pass  through  the 
collodion  membrane  and  fall  in  beautiful  ribbon-like  streams  to  the 
bottom,  while  a  considerable  time  will  elapse  before  the  same  phenome- 
non will  be  apparent  in  the  vessel  containing  the  parchment  paper. 

Ultrafiltration  and  Osmometer.     Small  collodium  sacks  have  been 

employed  for  some  time  by  bacteriologists  for  filtering  microbes.    G.  Mal- 

fitano  *  was  the  first  to  point  out  that  these  sacks  could  be  used  with 

advantage  in  the  investigation  of  colloids.     The  device  is  now  in  gen- 

*  G.  Malfitano:  Compt.  rend.,  139,  1221  (1904). 


36 


CHEMISTRY  OF  COLLOIDS 


eral  use.  Collodion  membranes  are  not  only  suitable  for  dialysis,  but 
they  also  make  it  possible  to  separate  the  colloid  from  the  larger  portion 

of  the  liquid.  It  is  therefore  possible  to 
measure  the  conductivity  of  the  filtrate 
and  the  residue  separately.  There  is  the 
further  advantage  that  these  membranes 
may  be  employed  to  determine  the  osmotic 
pressure  of  the  hydrosol  against  its  filtrate, 
and  thus  enable  us  to  take  another  step 
forward  in  the  investigation  of  colloidal 
chemistry. 

The  preparation  of  collodion  membranes 
is  comparatively  simple.  According  to 
Malfitano  25  gms.  of  collodion  wool  are 
covered  with  absolute  alcohol  and  the  vol- 
ume is  made  up  to  one  liter.  The  liquid 
must  be  prepared  some  time  in  advance. 
A  thin  film  of  collodion  is  formed  on  the 
outside  of  a  clean  test  tube  by  dipping  the 
latter  in  the  solution.  The  film  is  allowed 
to  dry,  dipped  in  water,  and  the  entire 
process  repeated  several  times  in  order  to 
obtain  a  membrane  of  sufficient  thickness. 
By  gentle  twisting  the  membrane  may  be 
removed  from  the  test  tube  and  fitted  over 
an  open  tube  having  a  diameter  of  the 
same  dimensions  as  the  test  tube.  The 
little  sack,  which  looks  like  the  finger  of 
a  glove,  is  filled  with  water  and  the  latter 
filtered  through  by  pressure.  The  sack  is 
thus  washed  and  tested  at  the  same  time. 
A  more  convenient  method  is  described  by 
Lillie.  A  concentrated  solution  of  collodion 
is  introduced  into  a  small  flask,  the  excess 
of  which  is  poured  off.  The  membrane 
formed  on  the  inside  of  the  walls  is  now 
FIG.  7.  Osmotic  cell.  treated  with  water  whereby  the  film  can 

be  easily  removed  from  the  glass.  The  sack  is  next  bound  by  means 
of  rubber  bands  to  a  long  tube,  the  end  of  which  is  closed  by  a  rubber 
stopper. 

The  membranes  are  tested  by  filling  them  with  a  colloidal  gold  solu- 
tion.    The  presence  of  holes  or  other  imperfections  is  betrayed  by  the 


PROPERTIES  OF  COLLOIDS  37 

red  color  of  the  filtrate.  In  the  author's  laboratory  collodion  mem- 
branes (B),  Fig.  7,  are  prepared  for  osmotic  pressure  measurements  by 
filling  a  test  tube  with  the  concentrated  solution  of  collodion.  For 
filtration  thinner  collodion  films  are  desirable.  These  are  prepared 
by  pouring  dilute  solutions  of  collodion  over  glass  plates.  The  solu- 
tion is  made  from  200  cc.  of  a  6  per  cent  solution  of  collodion  to  which 
is  added  200  cc.  ether  and  500  cc.  alcohol.  As  soon  as  the  films  are  no 
longer  sticky  they  are  treated  with  water  ^whereby  they  are  easily 
loosened  from  the  glass.  They  are  next  laid  on  a  piece  of  paper  and 
placed  in  a  filter  funnel.  In  this  form  they  are  suitable  for  the  filtra- 
tion of  either  hydrosols  or  very  finely  divided  precipitates.  That  they 
are  well  adapted  for  quantitative  analysis,  such  as  microanalysis,  has 
been  shown  by  Zsigmondy,  Wilke-Doerfurt,  and  v.  Galecki.* 

The  outer  vessel  of  the  osmometer,  Fig.  7,  has  a  side  tube  that  is 
graduated  in  cubic  centimeters.  The  long  tube  E  goes  through  a 
rubber  stopper  into  the  inner  tube.  The  scale  C  is  divided  into  milli- 
meters and  permits  the  reading  of  the  difference  of  level  in  the  two 
tubes.  In  order  to  prevent  evaporation  as  far  as  possible  D  is  closed 
by  a  rubber  stopper  through  which  passes  a  capillary  tube. 

Duclaux  f  has  used  this  apparatus  to  determine  whether  there  is  any 
change  taking  place  in  the  colloid.  Part  of  the  solution  is  filtered 
through  a  special  collodion  filter.  The  remainder  of  the  solution  is 
put  into  the  sack  B  while  the  filtrate  is  contained  by  the  outer  vessel  A. 
If  the  osmotic  pressure  is  greater  in  the  inner  solution  the  liquid  will 
rise  in  E.  After  some  time  the  height  of  the  column  remains  constant. 
The  level  is  now  restored  to  its  former  position  by  means  of  pressure 
in  E.  If  there  has  been  no  change  in  the  colloid  the  liquid  should  again 
rise  in  the  tube  until  it  attains  its  former  maximum.  Similar  forms  of 
osmometers  have  been  described  by  Lillie,  Hiifner,J  and  by  W.  Biltz.§ 

Bechhold's  Ultrafiltration.  —  For  the  ultrafiltration  of  colloids  in 
large  quantities  an  apparatus  constructed  by  Bechhold  If  is  well  adapted. 
With  this  apparatus  it  is  possible  to  separate  colloids  from  one  another 
according  to  the  size  of  the  pores  of  the  filter.  For  a  filter  Bechhold 
employs  strong  paper  that  has  been  impregnated  with  hardened  gelatin 
or  collodion  treated  with  glacial  acetic  acid.  Accordingly  as  the  solu- 
tion is  dilute  or  concentrated  a  filter  is  obtained  with  varying  perme- 
ability. The  size  of  the  pores  may  be  varied  within  certain  limits. 

*  R.  Zsigmondy,  E.  Wilke-Dorfurt  und  A.  v.  Galecki,  Ber.,  46,  579-582  (1912). 

t  J.  Duclaux:  page  33;  Journ.  de  chim.,  7,  430  (1909). 

j  Hufner:  See  Chapter  XII,  page  234. 

§  W.  Biltz:  page  33. 

H  H.  Bechhold:  Zeit.  f.  phys.  Chemie,  60,  257-318  (1907);  64,  328-342  (1908). 


38 


CHEMISTRY  OF  COLLOIDS 


The  impregnation  of  the  filter  is  carried  out  in  a  special  vessel,  which 
permits  the  treatment  of  a  large  number  simultaneously.  The  pre- 
pared filters  are  hardened  if  necessary  and  then  washed  with  water. 

Fig.  8  is  a  diagram  of  Bechhold's  apparatus.  P  is  a  plate  that  sup- 
ports the  filter  funnel  Tr  inside  the  cylinder  H .  The  filter  Fi  is  stretched 
across  a  piece  of  wire  gauze  N.  Above  and  below  the  filter  with  the 
gauze  suitable  calking  G  is  arranged.  The  same  material  in  the  form 
of  a  ring  washer  is  placed  between  the  edges  of  the  funnel  and  its  lid  D. 
The  screw  ring  is  for  holding  the  cover  securely  in  place.  The  solu- 
tion to  be  filtered  is  poured  into  the  funnel  and  pressure  is  applied. 
Comparatively  small  pressures  are  necessary  for  the  filtration  of  col- 
loids. 0.4  atmos.  is  sufficient  to  separate  the  liquid  medium  from  the 
colloid  in  the  case  of  dilute  hemoglobin  solutions.  Hemoglobin  and 
serum  albumin  may  be  thus  concentrated  to  a  greasy  paste  which  will 


FIG.  8.    Bechhold's  ultrafiltration  apparatus. 

redissolve  in  water.  In  order  to  measure  the  approximate  size  of  the 
pores  in  the  filter,  Bechhold  employed  a  hemoglobin  solution.  This 
solution  will  be  retained  by  the  less  permeable  membranes  but  will  pass 
through  those  having  large  pores.  For  further  information  about 
Bechhold's  experiments  the  original  articles  must  be  consulted. 

The  separation  of  colloids  by  ultrafiltration  is  of  very  great  interest. 
In  this  manner  Bechhold  was  able  to  filter  hemoglobin  off  from  a  mix- 
ture of  that  colloid  in  water  with  Prussian  blue.  The  muddy  green 
mixture  gave  a  red  filtrate  if  a  filter  with  large  pores  was  used,  and  a 
colorless  filtrate  if  the  filter  was  more  impermeable.  Another  interest- 
ing case  is  the  separation  of  albumoses.  Until  this  method  was  discov- 
ered the  decomposition  products  of  proteids  could  be  separated  only 
by  fractional  precipitation  with  salt  solutions  of  different  concentra- 
tions. See  Chapter  XII.  Bechhold  was  able  by  use  of  filters  differ- 
ing in  permeability  to  separate  protalbumoses  from  deuteroalbumoses, 
the  latter  going  into  the  filtrate.  It  is  remarkable  that  by  fractional 


PROPERTIES  OF  COLLOIDS  39 

precipitation  protalbumoses  fall  out  of  solution  when  the  concentra- 
tion of  the  ammonium  sulfate  used  for  that  purpose  is  only  one-quarter 
to  one-half  the  saturation  value,  while  a  much  stronger  solution  of 
ammonium  sulfate  must  be  employed  in  order  to  coagulate  the  deutero- 
albumoses.  Globulin,  which  requires  a  certain  amount  of  alkali  salts 
for  solution,  will  coagulate  during  ultrafiltration  into  a  white  opaque 
mass  that  redissolves  in  a  solution  of  sodium  chloride. 

Very  interesting  observations  with  mixtures  of  colloids  and  crystal- 
loids have  been  made  by  Bechhold.  A  mixture  of  colloidal  albumin 
and  methylene  blue  was  filtered  and  the  discovery  made  that  the  par- 
ticles of  albumin  had  taken  up  the  methylene  blue  in  a  manner  simi- 
lar to  the  behavior  of  animal  and  vegetable  tissues.  A  very  pretty 
experiment  illustrates  this  fact.  A  solution  of  methylene  blue  to 
which  some  albumin  has  been  added  will  not  dye  wool  to  the  same 
extent  as  will  a  pure  solution  of  the  dyestuff  alone. 

Table  2  contains  a  list  of  substances  arranged  in  the  order  in  which 
they  will  be  retained  by  filters  of  decreasing  permeability. 

TABLE  2 

Suspensions  One  per  cent  gelatin  solutions 

Prussian  blue  1  per  cent  hemoglobin  solution  (mol. 

Platinum  sols  (Bredig)  wt.  ca.  16,500) 

Colloidal  iron  oxide  Serum  albumin  (mol.  wt.  5000  to  15,000) 

Casein  (in  milk)  Diphtheria  toxin 

Colloidal  arsenious  sulfide  Protalbumoses 

Colloidal  gold  solutions  No.  4  (about  Colloidal  silicic  acid 

40  MM)  Lysalbinic  acid 

Bismon    (colloidal   bismuth   oxide   ac-  Deuteroalbumoses  (A) 

cording  to  Paal)  Deuteroalbumoses  (5).  (Mol.  wt.  ca. 
Lysargin  (colloidal  silver  according  to  2400) 

Paal)  Deuteroalbumoses  (<7) 

Kollargol  (colloidal  silver  according  to  Litmus 

v.  Heyden,  ca.  20  MM)  Dextrin  (mol.  wt.  ca.  965) 

Gold  solution  *  No.  0  Crystalloids 

It  should  be  noted,  however,  that  this  table  is  arranged  with  regard 
to  solutions  that  Bechhold  employed  and  does  not  necessarily  represent 
the  correct  relations  of  the  size  of  particles  in  colloidal  solutions  in 
general.  In  fact  Prussian  blue,  colloidal  iron  oxide,  and  many  other 
colloids  may  be  prepared  with  particles  varying  so  in  size  that  the 
relation  to  hemoglobin  would  be  quite  different  from  that  indicated  by 
the  table.  Finally  ultrafiltration  is  affected  not  only  by  the  size  of 
the  particles,  but  also  by  other  factors,  such  as  adsorption,  electric 
charge,  etc. 

*  The  position  of  this  solution  in  relation  to  hemoglobin  has  not  yet  been  defi- 
nitely determined. 


40  CHEMISTRY  OF   COLLOIDS 

Movements  of  the  Ultramicrons 

One  of  the  characteristic  properties  of  colloidal  solutions  is  the  more 
or  less  energetic  movement  of  the  particles.  The  closer  study  of  this 
movement  has  been  made  possible  by  the  ultramicroscope.  The  move- 
ment of  very  small  particles  can  be  seen  with  the  ordinary  microscope, 
and  has  been  known  since  1827.  It  was  discovered  by  a  botanist, 
Robert  Brown,*  and  has  been  named  after  him.  It  has  since  been 
thoroughly  investigated  by  a  large  number  of  scientists.  A  short 
bibliography  relative  to  the  earlier  experiments  will  be  found  in  Leh- 
mann's  Molekularphysik,  1,  264  (1867). 

Many  theories  have  been  evolved  to  explain  this  phenomenon. 
Regnault  f  assumed  that  the  heating  of  the  particles  on  one  side  more 
than  the  others  caused  small  convection  currents  and  this  accounted 
for  the  vibrating  movement  of  the  particles.  Wiener  t  concluded  that 
it  was  a  movement  peculiar  to  the  liquid  state,  imperceptible  of  itself 
but  rendered  manifest  by  the  colloidal  particles.  According  to  Quincke  § 
the  Brownian  movement  is  due  to  the  spreading  out  of  layers  of  liquid  over 
the  surface  of  the  particles.  A  really  useful  theory  was  first  arrived  at 
by  the  application  of  the  kinetic  hypothesis.  Wiener,  Cantoni,  Renard, 
Boussinesq,  and  Gouy  have  all  explained  the  movement  on  the  grounds 
of  collisions  between  the  particles  and  the  molecules  of  the  liquid.  It 
remained,  however,  for  the  theoretical  considerations  of  Einstein  If  and 
v.  Smoluchowski  1 1  to  point  out  the  way  in  which  the  experimental  proof 
could  be  obtained.  The  proof  has  been  given  by  The  Svedberg,**  Ehren- 
haft,ft  Perrin,J{  and  argues  for  the  justification  of  the  application  of  the 
kinetic  theory. 

Many  observations  had  already  shown  the  fallacies  in  the  older  the- 
ories. For  instance  it  had  been  shown  that  the  Brownian  movement 

*  R.  Brown:  Poggendorffs  Annalen  d.  Phys.  u.  Chem.,  14,  294-313  (1828). 

t  J.  Regnault:  Journ.  de  Pharm.  (3),  34,  141  (1858). 

j  Chr.  Wiener:  Poggendorffs  Annalen  d.  Phys.  u.  Chem.,  118,  79-94  (1863). 

§  G.  Quincke:  Verb.  d.  Ges.  D.  Naturf.  u.  Arzte.  Diisseldorf,  26-29  (1898); 
Beibl.  z.  d.  Annalen  de  Phys.  u.  Chem.,  23,  934-937  (1899). 

H  A.  Einstein:  Drudes  Annalen  d.  Phys.  (4),  17,  549-560  (1905);  19,  371-381 
(1906);  Zeit.  f.  Elektrochemie,  14,  235-239  (1908). 

||  M.  v.  Smoluchowski:  Drudes  Annalen  d.  Phys.  (4),  21,  756-780(1906);  26, 
205-226  (1908). 

**  The.  Svedberg:  Studien  zur  Lehre  von  den  kolloiden  Losungen,  125-160. 
Up&ala  (1907). 

ft  F.  Ehrenhaft:  Sitzungengsber.  d.  Akad.  d.  Wiss.  Wien,  116,  Ha,  1139-1149 
(1907). 

n  J.  Perrin:  Annales  de  Chim.  et  de  Phys.  (8),  18,  5-114  (1909).  J.  Donau: 
Kolloidchem.  Beihefte,  1,  1-258  (1910). 


PROPERTIES  OF  COLLOIDS  41 

was  independent  of  external  influences.  A  good  suspension  of  rubber 
can  be  boiled  for  a  week  at  a  time,  and  if  cooled  there  will  be  no  change 
in  the  Brownian  movement.  The  heat  rays,  any  specific  color,  or  all 
the  colors  may  be  cut  off;  or  the  intensity  of  the  illumination  may  be 
varied  a  thousand-fold  without  changing  the  movement  in  the  slightest. 
From  this  it  may  be  concluded  that  the  cause  is  not  local  changes  of 
temperature,  as  has  been  claimed  by  many  scientists,  but  is  a  perma- 
nent phenomenon  independent  of  exterior  circumstances. 

Investigations  with  the  ultramicroscope  *  have  established  the  fol- 
lowing facts  :  — 

1.  The  Brownian  movement  is  inversely  proportional  to  the  size  of 
the  particles.     In  fact  with  particles  of  very  small  dimensions  the  move- 
ments may  become  so  violent  as  to  assume,  from  all  appearances,  an 
entirely  different  character.* 

2.  The  movement  is  independent  of  the  heat  rays;  these  are  cut  off 
entirely  before  the  ray  passes  into  the  cuvette  of  the  ultramicroscope. 

3.  The  movement  is  not  caused  by  evaporation  because  the  observa- 
tions were  made  in  a  closed  vessel  completely  filled  with  the  liquid. 

4.  The  movement  is  independent  of  the  direction  of  the  rays,  of  the 
intensity  of  the  illumination,  and  of  its  duration. 

5.  The  particles  appear  to  affect  one  another,  as  the  movement  is 
somewhat  less  energetic  when  the  solution  is  diluted. 

Theory  of  the  Motion.  —  The  theory  of  Einstein  f  (v.  SmoluchowskiJ 
came  independently  to  the  same  result)  assumes  no  difference  between 
true  molecules  and  particles  suspended  in  the  same  medium.  The 
particles  behave  as  if  they  were  true  gas  molecules  with  normal  kinetic 
energy  but  a  much  shorter  free  path.  The  following  is  the  Einstein 
formula. 


A  _*\ 

* 

where 

A  is  the  amplitude  (in  the  direction  of  the  x  axis), 
t  is  the  corresponding  time, 
.R  is  the  ordinary  gas  constant, 
T  is  the  absolute  temperature, 
N  is  the  number  of  molecules  in  one  gram  mol, 
77  is  the  viscosity  of  the  medium, 
r  is  the  radius  of  the  particles  (average). 

v.  Smoluchowski  from  quite  other  assumptions  came  to  the  same 
formula  except  that  another  integer  is  introduced. 

*  R.  Zsigmondy:    Zur  Erkenntnis  der  Kolloide,  106-111  (1905). 
t  1.  c.  t  1.  c. 


42  CHEMISTRY  OF  COLLOIDS 

The  experimental  proof  of  the  formula  is  possible  from  several  direc- 
tions. 

1.  It  is  possible  to  determine  how  the  amplitude  changes  with  the 
temperature.     This  has  been  recently  done  by  Seddig  *  working  with 
solutions  of  tin. 

2.  It  is  also  possible  to  determine  how  A  changes  with  the  size  of 
the  particles. 

Svedberg  f  investigated  the  formula  by  two  other  methods.  He 
determined  whether  or  not  the  amplitude  measured  agreed  with  that 
calculated  from  the  formula.  By  a  suitable  contrivance  on  the  ultra- 
microscope  he  was  able  to  resolve  the  irregular  vibratory  motion  into 
waves.  By  measuring  the  wave  length,  the  amplitude  and  the  rate  of 
flow  of  the  liquid  through  the  cuvette  he  found  the  time.  The  values 
thus  obtained  are  somewhat  larger  than  those  calculated.  It  is  prob- 
able that  wave  length  was  underrated  where  the  amplitude  was  large. 

Svedberg  also  succeeded  in  confirming  the  relations  represented  by 
following  formula  : 


—  —  =  const. 
t 

It  will  be  readily  seen  that  this  formula  comes  directly  from  the 
original  one  if  T  and  r  are  kept  constant. 

Of  importance  is  the  conclusion  arrived  at  by  experimental  methods 
that  the  Brownian  movement  is  independent  of  the  electric  charge. 
The  claim  that  the  motion  is  due  to  an  interchange  of  charges  between 
ions  and  the  particles  is  therefore  untenable. 

Very  important  in  relation  to  the  motion  of  larger  particles  are  the 
researches  of  Perrin  J  on  gamboge  and  mastic  solutions.  They  show 
that  the  spontaneous  sedimentation  under  the  influence  of  gravity  is 
governed  by  the  same  laws  that  obtain  in  regard  to  the  molecules  of 
the  atmosphere.  Just  as  the  density  of  the  gas  changes  with  the 
height  according  to  an  exponential  law,  so  does  the  density  of  the  par- 
ticles in  suspension  change  with  the  depth  of  the  medium.  For  gases 
the  concentration  becomes  one-half  for  every  3f  miles  increase  in 
height;  while  in  the  case  of  suspended  particles  the  distance  is  only  TV 
mm.  for  the  same  change  in  concentration.  Perrin  was  enabled  by  his 
results  to  calculate  the  kinetic  energy  of  a  single  particle,  and  found  it 
to  correspond  to  that  of  a  molecule.  By  measuring  the  change  of  place 
of  the  particles  every  30  seconds  Perrin  was  able  to  show  the  validity 

*  M.  Seddig:  Habilitationsschr.  d.  Akad.  Frankfurt  a.  M.  (1907);  Zeit.  f.  anorg. 
Chemie,  73,  360-384  (1912). 
t  Svedberg:  1.  c. 
$  Perrin:  1.  c. 


PROPERTIES  OF  COLLOIDS  43 

of  Einstein's  formula.  For  further  information  reference  must  be  made 
to  the  original  article.  Also  see  Mecklenburg.*  A  recent  article  by 
Svedberg  and  K.  Inouyef  has  shown  that  the  formula  of  Einstein  is 
applicable  in  the  case  of  colloidal  gold  solutions.  The  value  of  N 
determined,  6.2  X  1023,  agrees  exactly  with  that  given  by  Rutherford, 
and  very  well  with  Perrin's,  7.1  X  1023. 

The  most  important  results  may  be  summed  up  as  follows.  Sus- 
pended particles  have  the  same  kinetic  energy  as  gas  molecules.  The 
particles  exert  an  osmotic  pressure  on  a  membrane  impermeable  to 
them.  This  osmotic  pressure  is  dependent  upon  the  temperature  and 
upon  the  number  of  particles  in  the  unit  volume.  The  last  named  fact 
applies  to  particles  of  all  sizes.  While  the  osmotic  pressure  is  too  small 
for  measurement  in  colloidal  suspensions,  it  is  much  greater  than  the 
experimental  error  in  the  case  of  hydrosols  where  the  particles  are  very 
small,  and  can  therefore  be  determined  with  some  degree  of  accuracy. 

Osmotic  Pressure  and  the  Number  of  Particles 

It  has  been  repeatedly  proved  experimentally  that  the  osmotic  pres- 
sure of  a  colloidal  solution  is  proportional  to  the  number  of  particles. 
In  case  the  colloid  is  free  from  electrolytes  we  have,  therefore,  an- 
other means  of  determining  the  number  of  particles.  The  proper- 
ties of  a  colloidal  solution  change  gradually  with  a  decrease  in  the 
number  of  particles.  Amicrons  unite  with  each  other  to  form  submi- 
crons  and  finally  the  entire  colloid  coagulates.  If  such  an  occurrence 
is  followed  in  an  osmometer  the  pressure  sinks  to  zero  during  the  coagu- 
lation, or  quite  often  before  it  takes  place.  Many  colloids  are  very 
stable  and  manifest  the  same  osmotic  pressure  for  months.  Under  the 
ultramicroscope  these  show  no  change  in  the  particles. 

The  influence  of  electrolytes  on  albumin  and  gelatin  has  been  deter- 
mined by  Lillie.J  The  concentration  of  the  electrolyte  was  made  the 
same  in  both  the  outer  and  the  inner  liquids.  The  hundreds  of  experi- 
ments that  he  carried  out  all  go  to  show  that  electrolytes  having  a 
precipitating  effect  on  the  colloid  diminish  the  osmotic  pressure. 
In  cases,  on  the  other  hand,  where  the  electrolyte  causes  swelling 
or  distention,  and  subsequent  dissolution  of  the  colloid,  the  osmotic 
pressure  is  increased.  The  law  of  Hofmeister  and  Paal  was  con- 
firmed, which  states  that  salts  having  an  intense  precipitating  effect 

*  W.  Mecklenburg:  Die  experimentelle  Grundlegung  der  Atomistik.  Jena 
(1910). 

t  The  Svedberg  und  K.  Inouye:  Arkiv.  for  kemi,  mineral,  och  geo'l.  4,  No.  19; 
Zeit.  f.  phys.  Chemie,  73,  547-556  (1910). 

J  Lillie:  1.  c. 


44  CHEMISTRY   OF  COLLOIDS 

diminish  the  osmotic  pressure  more  than  do  those  which  cause  little 
precipitation. 

The  dependence  of  the  osmotic  pressure  upon  the  temperature  is  of 
interest.  But  here  the  same  law  obtains,  namely  that  any  circumstances 
causing  the  particles  to  unite  diminish  the  pressure.  With  colloidal 
iron  oxide  a  rise  of  temperature  lessens  the  osmotic  pressure.  With 
many  other  colloidal  substances,  such  as  gelatin,  raising  the  tempera- 
ture increases  the  number  of  particles  and  therefore  the  osmotic  pres- 
sure becomes  greater. 

From  the  evidence  we  have,  showing  that  the  osmotic  pressure  varies 
with  the  size  of  the  particles,  we  may  conclude  that  colloidal  particles 
cause  the  pressure.  It  is  undeniable,  of  course,  that  any  electrolyte 
present  as  impurity  will  increase  the  values  obtained  by  measurement. 

Electrical  Properties  of  Colloids 

Under  the  influence  of  a  fall  of  potential  almost  all  colloids  migrate 
either  to  the  cathode  or  to  the  anode.  The  phenomenon  of  electrical 
migration  of  colloids  is  closely  associated  with  that  of  electro-osmosis. 
This  latter  phenomenon  may  be  defined  as  the  passage  of  liquids  through 
membranes  under  the  influence  of  a  fall  of  electrical  potential.  When 
the  particles  move  through  the  solution  the  phenomenon  is  called  cata- 
phoresis.  When  the  particles  remain  stationary  and  the  liquid  moves, 
the  term  endosmosis  is  applied.  The  earliest  observations  on  these 
phenomena  were  made  by  Picton  and  Linder,*  Coehn,  f  Lottermoser,t 
and  Wiedemann.§ 

Suspended  particles  in  contact  with  a  liquid  medium  take  on  a  charge 
either  positive  or  negative,  as  has  been  shown  by  Quincke.lf  These 
charges  determine  the  direction  in  which  the  particles  will  travel  in  the 
current.  With  regard  to  the  nature  of  the  charge,  a  law  enunciated  by 
Coehn||  gives  us  information.  Substances  with  a  high  dielectric  con- 
stant become  charged  positively  toward  those  with  a  lower.  The 
dielectric  constant  of  water  is  80,  which  is  very  high,  and  therefore 
most  colloids  are  charged  negatively  in  pure  water;  in  other  words  the 
particles  move  towards  the  anode.  On  the  other  hand  the  dielectric 
constant  of  turpentine  is  low,  therefore  the  particles  suspended  in  it 

*  H.  Picton  and  S.  E.  Linder:  Journ.  Chem.  Soc.,  61,  148-172  (1892). 

t  A.  Coehn:  Zeit.  f.  Elektrochemie,  4,  63-67  (1897). 

$  A.  Lottermoser  und  E.  v.  Meyer:  Journ.  f.  prakt.  Chemie  (2),  66,  241-247 
(1897). 

§  G.  Wiedemann:  Poggendorffs  Annalen  d.  Phys.  u.  Chem.  87,  321-352  (1852). 
Die  Lehre  von  der  Elektrizitat,  (2  Aufl.),  1,  993-1019  (1893). 

f  G.  Quincke:  Poggendorffs  Annalen  d.  Phys.  u.  Chem.,  113,  513-598  (1861). 

||  A.  Coehn:  Wiedemanns  Annalen  d.  Phys.  (N.  F.),  64,  217-232  (1898). 


PROPERTIES  OF  COLLOIDS 


45 


assume  a  positive  charge  and  move  toward  the  cathode.  The  law 
obtains  quantitatively  only  for  substances  having  a  very  low  conduc- 
tivity. Coehn  and  Raydt  *  found  that  dielectric  constants  could  be 
calculated  from  the  height  of  the  electro-osmotic  column. 

The  influence  of  the  presence  of  electrolytes  is  of  first  importance  in 
the  nature  of  the  charge  on  the  particles.  The  adsorption  of  ions  and 
other  chemical  reactions  are  deciding  factors.  The  following  table 
shows  the  charges  that  are  usually  found  on  colloidal  particles,  provided 
that  the  customary  methods  of  preparation  are  employed,  and  the 
colloids  purified  by  dialysis.  Under  special  circumstances  the  charges 
may  be  just  the  opposite  of  that  given  in  the  table. 


TABLE  3 


Migration  toward  cathode 


Charge  of  the  Particles 


Colloidal  iron  oxide 

1        cadmium  hydroxide 

'        aluminium  hydroxide 

'        chromium  oxide 

'        titanic  acid 

1        thorium  oxide 

1        zirconium  oxide 

4        cerium  oxide 

Basic  dyestuffs,  dissolved  either  as  a 
colloid  or  a  crystalloid 


Colloidal  gold,  silver,  platinum 
"       sulfur 

"       arsenious  sulfide 
"       antimonious  sulfide 
"       cupric  sulfide 
"       lead  sulfide 
"       cadmium  sulfide 
"       mastic 

"       gamboge,  gum  arabic 
"       soluble  starch 
"       silicic  acid 
"       stannic  acid 
"       molybdenum  blue 
"       tungsten  blue 
"       vanadium  pentoxide 

Acid   dyestuffs    (colloidal   as    well   as 
crystalloidal) 

The  hydrogen  and  hydroxide  ions  are  especially  effective  in  deter- 
mining the  nature  of  the  charge.  Hardy  f  found  that  albumin  had  no 
perceptible  charge  in  pure  water,  but  that  traces  of  alkalies  or  acids 
caused  the  albumin  to  go  to  the  anode  or  to  the  cathode  respectively. 
Perrin  t  found  the  same  rule  to  hold  for  many  suspended  powders,  and 
many  hydrosols,  such  as  albumin  solutions.  Pauli§  corroborated  Per- 
rin's  results  on  proteids. 

A  remarkable  fact  was  discovered  by  Coehn, If  namely  that  crystal- 

*  A.  Coehn  und  U.  Raydt:  Annalen  d.  Phys.  (4),  30,  777-804  (1909). 
t  W.  B.  Hardy:  Journ.  of  Physiol.,  24,  288-304  (1899);  Zeit.  f.  phys*  Chemie,  33, 
385-400  (1900). 

t  J.  Perrin:  Compt.rend.,  136,  1388-1391  (1903);  137,  513-547,  564-566  (1903). 
§  W.  Pauli:   Hofmeisters  Beitrage  z.  chem.  Phys.  u.  Path.,  7,  531-547  (1906). 
If  A.  Coehn:  Zeit.  f.  Elektrochemie,  16,  652-654  (1909). 


46 


CHEMISTRY  OF  COLLOIDS 


loidal  solutions  of  non-electrolytes,  such  as  sugar,  migrate  toward  the 
anode  in  dilute  alkali  and  toward  the  cathode  in  dilute  acid  solution. 

The  above  laws  are  not  of  general  application,  however.  Many 
colloids  refuse  to  change  their  direction  with  the  nature  of  the  solution. 
To  this  class  belongs  silicic  acid  which  always  migrates  toward  the 
anode  unless  a  concentrated  acid  solution  is  employed. 

Although  there  exists  a  very  great  similarity  between  the  behavior 
of  colloids  and  suspended  particles  in  the  electric  current  it  should  not 
be  overlooked  that  the  former  manifest  an  even  greater  similarity  to 
the  migration  of  ions.  True  suspensions  may  be  discharged  at  both 
poles,  so  that  colloids  seem  to  occupy  a  position  intermediate  between 
suspensions  and  crystalloids. 

Endeavors  to  determine  the  electrochemical  equivalent  of  colloids 
have  not  been  very  successful  in  most  cases  owing  to  the  confused  re- 
lations between  the  size  of  the  particles  and  the  charge.  Sometimes  it 
is  quite  impossible  because  the  substance  is  partly  colloid  and  partly 
crystalloid,  e.g.,  Benzopurple,  Congo  red.  Nevertheless,  the  amount  of 
electricity  on  a  definite  amount  of  colloid  has  been  determined.  In  all 
cases,  however,  where  the  particles  migrate  and  are  discharged  at  the 
electrodes,  electricity  must  be  transported.  Although  it  is  very  small 
in  comparison  to  that  of  solutions  of  electrolytes,  owing  to  the  com- 
paratively small  number  of  particles  involved,  it  has  been  often  deter- 
mined in  the  case  of  hydrosols. 

The  rate  of  migration  of  colloidal  particles  is  of  the  same  order  as 
that  of  suspensions  and  ions.  This  will  be  apparent  from  the  following 
table,  which  gives  the  rate  in  /*  per  second  for  a  potential  fall  of  one  volt 
per  centimeter. 

TABLE  4 


M 

Sec. 

Particles  having  a  diameter  of  35  M  

2.5  Quincke 

Colloidal  silver 

2  0  Svedberg 

«             « 

2  0  Burton 

"     (direct  current) 

3  2-3  8  Cotton  and 

"      (alternating  current)    . 

Mouton 
3.8         Cotton  and 

Colloidal  gold  (average)  ... 

Mouton 
4  0  v.  Galecki  * 

Anion  of  butyric  acid  

3.1  Ostwald,  Bredig 

Limit  of  mobility  of  organic  ions  having  a  high 
molecular  weight 

ca  2  0 

H+.. 

32  9 

OH-... 

18  0 

ci-  

6.8 

*  A.  v.  Galecki:  Zeit  f.  anorgan.  Chemie,  74,  174-206  (1912). 


PROPERTIES  OF  COLLOIDS 


47 


The  electrolysis  of  stannic  acid  hydrosol,  or  the  purple  of  Cassius  takes 
place  in  precisely  the  same  manner  as  that  of  the  crystalloids,  methyl- 
orange,  or  the  sodium  salts  of  dyestuffs.  In  both  cases  under  the  micro- 
scope the  separation  of  the  alkali  soluble  constituent  may  be  seen  at  the 
cathode.  Where  the  particles  are  small  enough  no  movement  can  be 
perceived  even  under  the  ultramicroscope.  Determinations  of  the 
migration  numbers  have  shown  that  in  one  case  seven  and  in  others 
ten  gram  mols  of  stannic  acid  were  transported  for  every  equivalent 
of  silver  deposited  in  the  voltmeter.  A  portion  of  the  stannic  acid 
was  deposited  on  the  anode. 

The  migration  of  colloids  frequently  differs  from  that  of  electro- 
lytes, in  that  coagulation  may  occur  before  the  electrode  is  reached,  as 
in  the  case  of  colloidal  gold  protected  by  gelatin;*  or  the  colloid  may 
migrate  toward  one  electrode  and  then  change  its  direction;  some 
albumins  and  many  colloidal  metals  show  this  disposition.  The  cause 
is  doubtless  due  to  the  fact  that  the  cathode  portion  becomes  alkaline, 
the  anode  acid  during  the  electrolysis.  The  translator  has  remarked 
this  behavior  on  the  part  of  many  colloids.  Finally  colloids  are  un- 
able to  penetrate  parchment  paper  even  with  the  help  of  the  current, 
becoming  discharged  and  coagulating  instead.!  Analogous  observations 
have  been  made  during  ordinary  elect roly sis. J  From  Table  4  it  is 
apparent  that  the  rate  of  migration  is  independent  of  the  size  of  the 
particles. 

Apparatus  for  the  Determination  of  the 
Direction  of  Migration.  —  The  direction  and 
the  rate  of  migration  of  ultramicrons  may  be 
determined  either  macroscopically  or  ultramicro- 
scopically.  The  U-shaped  tube,  Fig.  9,  used  by 
Coehn,§  lends  itself  very  well  for  macroscopic 
observations.  The  two  stopcocks  have  a  bore 
exactly  the  size  of  the  diameter  of  the  remainder 
of  the  tube.  The  apparatus  is  filled  with  the 
colloidal  solution  and  the  cocks  are  closed.  The 
two  end  portions  are  then  rinsed  out,  filled  with 
water,  and  the  electrodes  introduced  into  them. 
The  cocks  are  next  opened  and  the  current  turned 
on.  In  a  very  short  time  it  will  be  seen  that  the  boundary  between  the 
colloidal  solution  and  the  water  is  moving  either  toward  the  cathode  or 

*  J.  Billitzer:  Zeit.  f.  Elektrochemie,  8,  638-642  (1902);  Zeit.  f.  phys.  Chemie, 
45,  307-330  (1903). 

f  R.  Zsigmondy:  Liebigs  Annalen,  301,  36  (1898). 
j  W.  Ostwald:  Zeit.  f.  phys.  Chemie,  6,  71-82  (1890). 
§  A.  Coehn:  Zeit.  f.  Elektrochemie,  15,  653  (1909). 


FIG.  9. 


Coehn 's  appa- 
ratus. 


48  CHEMISTRY  OF  COLLOIDS 

toward  the  anode.*  In  the  case  of  ultramicrons,  easily  seen  in  the  ultra- 
microscope,  the  progress  of  the  individual  particles  may  be  followed  by 
means  of  this  instrument.  Cotton  and  Mouton  *  have  described  an 
apparatus  for  this  purpose,  and  given  directions  for  its  use.  A  some- 
what different  apparatus  has  been  constructed  by  Svedberg  f  by  which 
he  was  able  to  make  a  very  interesting  study  of  the  relation  of  charge 
to  motion  of  the  individual  particles. 

Charge  on  the  Particles.  —  There  is  no  longer  any  doubt  as  to  the 
existence  of  a  charge  on  the  particles,  and  it  therefore  behooves  us  to 
consider  the  origin  of  these  charges.  In  the  case  of  electrolytes  the 
assumption  is  made  that  the  molecule  dissociates  into  a  positive  and  a 
negative  ion.  That  holds  for  simple  salts  or  for  those  where  the  ions 
may  be  complex  and  have  a  high  molecular  weight.  If  these  complex 
ions  were  increased  in  mass  until  the  molecular  weight  is  several  thou- 
sand, the  solution  would  have  colloidal  properties.  This  is  doubtless  the 
way  in  which  the  electric  charge  originates  in  special  cases;  for  instance, 
dyestuffs  with  high  molecular  weights.  The  phenomenon  would  then  be 
quite  analogous  to  that  of  the  formation  of  ions.  In  a  large  number  of 
cases,  however,  this  explanation  would  meet  with  difficulties.  When 
non-conductors  are  under  consideration  the  charge  may  be  explained  by 
the  difference  in  the  dielectric  constant  of  the  disperse  phase  and  the 
disperse  medium,  as  already  mentioned  on  page  44.  In  other  cases 
the  existence  of  the  charge  must  be  explained  on  the  assumption  of  the 
adsorption  of  ions,  or  the  giving  off  of  ions  to  the  liquid.  Suppose  we 
take  for  example  colloidal  cadmium  made  by  Bredig's  method.  The 
individual  cadmium  particles  would  be  tiny  electrodes,  sending  off 
positively  charged  ions,  and  becoming  themselves  charged  negatively. 
Doubtless  the  conductivity  of  this  solution  would  be  greater  than  that 
of  water,  and  both  the  charged  particles  and  the  ions  would  take  part 
in  the  transportation  of  electricity.  The  amount  carried  by  each  of 
these  two  factors  would  depend  upon  the  number  of  individuals,  the 
charge,  and  the  rate  of  migration.  If  such  a  solution  were  electrolized 
cadmium  should  be  deposited  on  both  electrodes.  More  would  be  found 
on  the  anode  than  on  the  cathode,  because  the  mass  of  the  colloidal 
particles  is  much  greater  per  unit  charge  than  that  of  the  ions .  This  could 
probably  not  be  carried  out  in  practice  because  these  metals  with  a  high 
solution  tension  would  oxidize  rapidly,  and  the  oxides  generally  travel  in 
the  opposite  direction  to  that  of  the  particles  of  pure  metal. 

The  electric  charge  may  be  explained  in  the  case  of  royal  metals  also 

*  A.  Cotton  et  H.  Mouton:  Les  ultramicroscopes,  etc.,  144.     Paris  (1906). 
t  The.  Svedberg:   Studien  zur  Lehre  von  den  kolloiden  Losungen,  149.     Upsala 
(1907). 


PROPERTIES  OF  COLLOIDS  49 

by  the  adsorption  or  dissociation  of  ions.  During  Bredig's  process  the 
colloidation  takes  place  principally  at  the  cathode,  where  hydrogen  is 
copiously  discharged.  The  metal  particles  may  contain  a  great  deal 
of  free  hydrogen,  give  off  hydrogen  ion,  and  become  negatively  charged, 
just  as  occurs  when  a  hydrogen  electrode  is  placed  in  water.  This 
assumption  has  been  made  by  Billitzer.  The  acquisition  of  a  charge 
by  gels  through  peptisation  will  be  taken  up  in  Chapter  IV.  It  can 
be  explained  only  by  adsorption. 

The  fundamentals  of  the  electrical  theory  of  irreversible  colloids  have 
been  dealt  with  by  Hardy.*  To  him  belongs  the  credit  of  having  made 
clear  the  relation  between  the  nature  of  the  charge  and  the  precipita- 
tion of  colloids  by  electrolytes.  According  to  his  theory  it  is  the  elec- 
tric charge  on  the  particles  of  irreversible  colloids  that  determines  the 
stability.  If  the  charge  is  neutralized  by  the  addition  of  electrolytes,  the 
isoelectric  point  is  reached  and  the  particles  unite;  or,  in  other  words, 
coagulation  occurs.  That  colloids  sensitive  to  electrolytes  do  coagu- 
late in  the  neighborhood  of  the  isoelectric  point  has  been  repeatedly 
shown.  Such  ions  may  take  part  in  the  coagulation  which  have  a  charge 
opposite  to  that  of  the  colloidal  particles.  This  theory  has  thrown 
light  on  a  great  many  previously  unexplained  relations.  It  was  not 
difficult  to  see  that  the  origin  of  the  charge  on  the  particles  was  due 
either  to  a  specific  partition  coefficient  between  the  particles  and  the 
medium,!  to  the  dissociation  of  ions  by  the  particles, J  or  to  the  ad- 
sorption of  one  ion  more  than  the  other.§ 

Discharge  of  the  Particles.  —  The  gradual  discharge  of  the  particles 
may  be  followed  macroscopically  (Burton), If  in  a  Nernst-Coehn  appa- 
ratus, or  ultramicroscopically  according  to  Svedberg.  By  the  latter 
method  it  is  possible  to  show  whether  the  charge  has  any  effect  on  the 
Brownian  movement  or  not.  Svedberg  [|  elicited  that  the  movement 
is  independent  of  the  presence  of  a  charge.  He  employed  colloidal  silver 
and  aluminium  sulfate  in  his  work,  and  found  that  a  few  hundred  thou- 
sandths of  one  per  cent  of  the  sulfate  were  sufficient  to  render  the  silver 
particles  neutral  toward  the  medium.  The  vibration  had  an  amplitude 
of  2.2  to  2.25  AI  regardless  of  whether  the  particles  were  neutral,  charged 
positively,  or  negatively.  From  this  it  can  be  at  once  concluded  that 
the  motion  is  not  due  to  electrical  causes. 

*  Hardy:  1.  c. 

t  G.  Bredig:  Anorganische  Fermente,  16.     Stuttgart  (1901). 
j  J.  Billitzer:  1.  c. 

§  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  page  165-169. 
1F  E.  F.  Burton:  Philos.  Magazine  (6),  11,  425-447  (1906);  12,  472-478  (1906); 
17,  583-597  (1909). 
||  Svedberg:  1.  c. 


50 


CHEMISTRY  OF  COLLOIDS 


The  isoelectric  point  is  known  by  the  fact  that  the  particles  unite  to 
form  large  complexes,  and  also  that  they  do  not  move  either  toward 
the  anode  or  cathode  when  a  fall  of  potential  exists  between  the  elec- 
trodes. 

The  exact  concentration  was  60.10~8  g.  aluminium  to  1  g.  of  the 
colloid  solution.  If  more  aluminium  sulfate  was  added  the  particles 
moved  in  the  opposite  direction.  The  gradual  discharge  of  the  parti- 
cles can  be  seen  from  the  following  table.  Recently  Galecki  *  has 
verified  the  results  of  Burton  and  Svedberg. 

TABLE  5 


Grams  of  A.\+++  in 
100  cc. 

for  a  potential 

S6C. 

fall  oi  1  V  per  cm. 

Grams  of  Al+++  in 
100  cc. 

-~  for  a  potential 

S6C. 

fall  of  1  V  per  cm. 

0 
Traces 
17.  10-6 
35.  10-6 

2.0 

1.38 
1.29 
1.03 

52.  10-6 
69.10-6 
87.HH 
173.  10-6 

0.26 
-0.42 
-0.61 
-1.56 

Magnitude  of  the  Charges  on  the  Particles.  —  In  order  to  explain 
the  coagulation  of  colloids  by  electrolytes,  Billitzer  f  assumed  that  the 
charge  on  the  particle  was  small  in  comparison  to  that  on  an  ion 
molecule.  One  ion  molecule  should  therefore  be  able  to  discharge 
a  large  number  of  colloidal  particles  and  cause  them  to  unite.  This 
hypothesis  does  not,  however,  agree  with  the  facts.  The  ultrami- 
croscopical  investigation  of  migration  has  revealed  the  fact  that  the 
rate  of  all  the  particles  in  a  given  hydrosol  is  almost  the  same,  and 
that  every  particle  is  only  gradually  discharged  on  the  addition  of 
an  electrolyte.  This  would  go  to  show  that  the  charge  on  an  indi- 
vidual particle  must  be  large  compared  to  that  on  a  univalent  ion 
molecule.  Were  this  not  the  case  the  addition  would  cause  a  sudden 
change  of  direction  of  the  particles  instead  of  a  gradual  diminution  of 
velocity.  Another  evidence  of  the  truth  of  this  deduction  regarding 
the  magnitude  of  the  charge  is  the  fact  that  the  velocity  of  the  par- 
ticles is  about  the  same  as  that  of  ion  molecules.  As  the  former  are 
very  much  larger  the  charge  must  be  correspondingly  greater  in  order 
to  produce  the  same  velocity. 

Assuming  Stokes' t  formula  the  magnitude  of  the  charge  on  a  par- 
ticle of  known  diameter  may  be  calculated  from  its  velocity.  This 

*  A.  v.  Galecki:  Zeit.  f.  anorg.  Chemie,  74,  174-206  (1912). 

t  J.  Billitzer:  Zeit.  f.  phys.  Chemie,  61,  129-166  (1905). 

t  Kirchhoff:  Vorlesungen  iiber  Mechanik.,  26  Vorlesung.     Leipzig  (1897). 


PROPERTIES  OF  COLLOIDS  51 

sort  of  calculation  indicates  that  the  charge  on  an  individual  particle 
of  medium  size  must  be  considerable.     In  Stokes'  formula 


e  is  the  charge,  17  the  viscosity  of  the  medium,  r  the  radius  of  the  par- 
ticles, and  u  the  velocity  of  the  particles  for  the  potential  fall  H.  For 

silver  particles  u  =  2-^-,  H  =      —  ,  i?  =  0.0105,  and  r  =  25  MM-     Sub- 

SGC.  Clll, 

stituting  these  values  in  the  equation  we  have 

-  6  X  3.14  X  1.05  •  10-2  X  25  -  10~7  X  20  •  1Q-6  _ 

6  —  ,  —  £to  i  *  1U 

31FZT 

electrostatic  units;  or  99  elementary  quantums,  where  the  value  of 
one  quantum  is  3.10~10  electrostatic  units.  The  charge  on  a  silver  par- 
ticle having  a  diameter  of  50  ^  and  a  velocity  of  2  /*  would  be  equal  to 
that  of  99  molecules  of  chloride  ion.* 

During  the  discharge  and  coagulation  of  colloids  by  electrolytes  the 
particles  adsorb  ions  having  a  charge  opposite  to  their  own.  The  pre- 
cipitating ion  is  carried  down  with  the  colloid  and  generally  cannot  be 
removed  by  washing.  This  ion  may,  however,  be  replaced  by  an 
equivalent  amount  of  another  ion  having  the  same  valence.  Many 
examples  of  this  have  been  observed  by  Picton  and  Linder,  Spring, 
Whitney  and  Ober,  and  others.  Picton  and  Linder  f  found  that  col- 
loidal arsenious  sulfide,  precipitated  by  a  solution  of  calcium  chloride, 
carried  down  some  of  the  calcium  and  left  an  equivalent  amount  of 
hydrogen  ion  in  solution.  In  other  words  the  solution  became  acid. 
The  calcium  in  the  precipitate  could  be  substituted  by  an  equivalent 
amount  of  barium.  The  explanation  of  this  phenomenon  will  be  given 
in  Chapter  IV.  Spring  J  made  a  similar  observation  with  mastic 
and  copper  sulfate.  Whitney  and  Ober  §  carefully  investigated  the 
precipitation  of  arsenious  sulfide  by  the  salts  of  the  alkaline  earth 
metals  and  found  that  a  fixed  amount  of  the  hydrosol  adsorbed  equiva- 
lent quantities  of  these  metals  1f  as  oxides  (or  hydroxides)  from  solutions 
of  their  salts. 

The  table  shows  that  almost  equivalent  quantities  of  the  metals  were 
adsorbed  and  carried  down.  Under  the  assumption  that  the  amount 
of  the  metal  found  in  the  precipitate  was  exactly  the  quantity  necessary 

*  Freundlich:  Kapillarchemie,  page  243.     Leipzig  (1909). 

t  H.  Picton  and  S.  E.  Linder:  Journ.  Chem.  Soc,,  67,  63-74  (1895). 

J  W.  Spring:  Archives  des  Sc.  Phys.  et  Natur.  (4),  10,  305-321  (1900). 

§  W.  R.  Whitney  und  J.  E.  Ober:  Zeit.  f.  phys.  Chemie,  39,  630-634  (1902). 

IT  Chapter  IV. 


52 


CHEMISTRY  OF  COLLOIDS 


to  neutralize  the  charge  on  the  particles,  and  that  nothing  was  lost 
during  washing,  etc.,  it  would  be  possible  to  calculate  the  magnitude 
of  the  charge  on  the  individual  particles  provided  the  average  mass 
was  known.  Freundlich*  has  recently  used  the  precipitation  values 
to  determine  the  valence  of  ions. 

TABLE  6 

Grams  adsorbed  by  100  cc.  of  the  colloidal  solution 


Found 

Calculated 

Ca  

(       0.0019 

|         0.0022 

Sr      

I       0  .  0020 
i       0.0036 

|       0  0049 

Ba.. 

(       0.0041 
0.0076 

0.0076 

K  

0.0036 

0.0043 

> 


Coagulation 

The  coagulation  by  electrolytes  has  been  studied  mostly  with  pure 
colloids  of  metals  or  with  sulfides.  The  laws  deduced  for  these  special 
cases  are  not  of  general  application.  These  systems  are  sensitive  to 
the  action  of  electrolytes.  The  cause  for  this  will  be  discussed  in  de- 
tail in  the  chapters  on  metal  colloids.  In  any  case  there  is  a  strong 
tendency  in  the  system  for  the  metal  and  the  water  to  reach  a  more 
stable  state  and  separate  from  each  other.  The  coagulation  of  the 
particles  is  a  spontaneous  irreversible  reaction  requiring  a  long  time  for 
its  completion,  and  a  slight  aid  by  the  electrolyte  causes  the  rate  to 
increase  very  greatly.  The  coagulation  may  occur  during  filtration,  or 
centrifugalization,  without  the  presence  of  the  electrolyte.  Contrary  to 
the  reversible  coagulation  of  clay  suspensions  that  of  the  metal  colloids 
is  quite  irreversible.  Nevertheless  there  are  points  of  semblance  in 
the  two  cases.  In  each  the  particles  are  discharged. 

In  order  to  precipitate  a  given  colloid  by  a  certain  electrolyte,  a  mini- 
mum concentration  must  be  exceeded.  Bodlander  f  has  called  this 
value  the  Schwellenwert.  This  minimum  is  not  easily  determined 
because  several  other  factors  are  involved;  such  as  the  rate  at  which 
the  addition  is  made,  the  method  of  stirring,  etc.  Freundlich  J  found 
that  a  given  volume  of  electrolyte  was  sufficient  to  cause  complete 
precipitation  if  added  quickly;  but  that  the  coagulation  was  only  par- 
tial if  the  addition  was  made  very  slowly.  If  the  operation  is  performed 

*  Zeit.  f.  phys.  Chemie,  80,  564  (1912). 

t  G.  Bodlander :*Neues  Jahrb.  f.  Min.  Geol.  usw.,  2,  147-168  (1893). 

J  H.  Freundlich:  Zeit.  f.  phys.  Chemie,  44,  129-160  (1903). 


PROPERTIES  OF  COLLOIDS 


53 


in  the  same  manner  each  time,  the  amounts  necessary  agree  very  well 
with  one  another. 

Rate  of  Coagulation. — When  a  sufficiently  large  excess  of  the  electro- 
lyte is  added  the  rate  of  coagulation  is  usually  very  great.  It  generally 
occurs  immediately  on  shaking,  On  the  other  hand,  sedimentation  is  in 
most  cases  slow.  Sometimes  coagulation  occurs  simultaneously  with 
chemical  reactions  between  the  colloid  and  the  electrolyte.  In  such  cases 
astonishing  results  are  often  obtained.  One  would  expect,  for  instance, 
that  colloidal  iron  oxide  and  hydrochloric  acid  would  unite  to  give  ferric 
chloride,  without  the  formation  of  a  precipitate.  If  the  experiment  is 
performed  with  a  somewhat  old  specimen  of  the  hydrosol  a  precipitate  is 
at  first  formed  which  redissolves  to  give  a  solution  of  ferric  chloride. 
Similarly  aqua  regia  and  potassium  cyanide  cause  colloidal  gold  to 
change  to  a  violet  color  although  both  dissolve  finely  divided  gold. 
Here,  as  in  the  previous  case,  coagulation  occurs  more  rapidly  than  the 
dissolution  of  the  substance.  A  great  many  other  analogous  cases  are 
known  in  which  a  precipitate  is  obtained  at  first  where  one  would  ex- 
pect immediate  dissolution. 

Valence  Relations.  —  The  comparison  of  the  precipitating  values  of 
different  electrolytes  found  by  Schulze,*  Prost,t  Picton  and  Linder  t 
gave  the  interesting  result  that  univalent  cathions  have  a  less  intensive 
effect  than  bivalent;  and  these  in  turn  were  not  so  effective  as  tri- 
valent  cathions.  This  can  be  easily  seen  from  Table  7  constructed  by 
Freundlich.§ 

TABLE  7 
A2S3  hydrosol,  charged  negatively,  concentration  =  1.857  gms.  per  liter 


Electrolyte. 

Precipitation 
value  in  mille- 
mols  per  liter. 

Electrolyte. 

Precipitation 
value  in  mille- 
mols  per  liter. 

KC2H3O2  .  . 

110  0 

MgCl2 

0.717 

Lid  

58.4 

MgSO4 

0.810 

NaCl  

51  0 

CaCl2 

0.649 

KNO3.  .. 

50.0 

SrCl2 

0.635 

KCL. 

49.5 

BaCl2.    . 

0.691 

K2SO4 

ftfC    ft 

ZnC12  

0.685 

2        
NH4C1 

42  3 

UO2(NO3)2  
Aids  

0.642 
0.093 

HC1 

30  8 

Al(NO3)3  

0.095 

Ce2  (S04)3 

0  OQ2 

' 

2 

*  H.  Schulze:    Journ.  f.  prakt.  Chemie  (2),  25,  431-452  (1882);    27,  320-332 
(1883). 

t  E.  Prost:  Bulletin  de  1'Acad.  Roy.  de  Belg.  (3),  14,  312-321  (1887). 
t  H.  Picton  and  S.  E.  Linder:  Journ.  Chem.  Soc.,  67,  63-74  (1895). 
§  H.  Freundlich:  Zeit.  f.  phys.  Chemie,  73,  385-423  (1910). 


54 


CHEMISTRY  OF  COLLOIDS 


By  precipitation  value  is  understood  that  concentration  of  the  salt 
lying  between  two  other  concentrations,  the  greater  of  which  causes 
the.  liquid  to  become  clear,  and  the  smaller  of  which  leaves  the  liquid 
slightly  turbid. 

Picton  and  Linder  *  demonstrated  that  the  precipitation  of  posi- 
tively charged  hydrosols,  such  as  colloidal  iron  oxide,  was  similarly 
affected  by  the  valence  of  the  anion.f  See  Table  8. 

TABLE  8 
Fe2O3  hydrosol,  charged  positively,  1.744  gms.  Fe(OH)3  per  liter 


Electrolyte. 

Precipitation  value 
mille-mols.  per 
liter. 

Electrolyte. 

Precipitation  value 
mille-mols.  per 
liter. 

KC1.. 

9.03 

K2SO4 

0  204 

KNO3.  . 

11.9 

MgSO4 

0  217 

BaCl2 

9f\A 

K2Cr2O7  

0.194 

2     

TABLE  9 


Electrolyte. 

Precipitation 
value  in  mille- 
mols  per  liter. 

Electrolyte. 

Precipitation 
value  in  mille- 
mols  per  liter. 

NaCl  

51.0 

Aniline  hydrochloride  

2.52 

Guanidine  nitrate  
Strychnine  nitrate 

16.4 
8  0 

Morphine  chloride  
New  fuchsine 

0.425 
0  114 

Freundlich  J  verified  these  results,  and  found  further  that  the  ad- 
sorption of  the  ions  played  a  part.  That  is,  strongly  adsorbed  cathions 
precipitated  negatively  charged  hydrosols,  and  vice  versa,  adsorption  of 
an  anion  caused  the  precipitation  of  positively  charged  hydrosols.  This 
will  be  apparent  from  Table  9. 

It  is  worthy  of  note  that  morphine  chloride  and  fuchsine  have  an  ab- 
normally low  precipitation  value,  although  each  forms  a  univalent 
cathion.  Moreover,  a  great  many  cathions  of  the  heavy  metals  have  a 
smaller  value  than  corresponds  to  their  valence,  so  that  this  law  is 
somewhat  limited  in  its  application. 

Freundlich  has  attempted  to  explain  the  law  by  use  of  adsorption 
isotherms,  Fig.  10.  Under  the  assumption  that  the  cathions  of  all 
light  metals  are  adsorbed  to  the  same  degree  it  follows  from  the  ad- 
sorption isotherms,  that  the  precipitation  values  of  univalent  ions 

*  H.  Picton  and  S.  E.  Linder:  Journ.  Chem.  Soc.,  87,  1906-1936  (1905). 
t  Exceptions  to  the  rule  are  given  in  Chapter  VII,  Colloidal  Iron  Oxide. 
1  H.  Freundlich:  Zeit.  f.  phys.  Chemie,  73,  385-423  (1910). 


PROPERTIES  OF  COLLOIDS 


55 


Bivalent  cathlon 


Trivalent  cathlon 


must  be  much  larger  than  those  of  bivalent  or  trivalent  ions.  In  order 
to  neutralize  the  charge  on  ultramicrons  equivalent  amounts  of  cathions 
must  be  adsorbed,  or  expressed  in 
mols,  three  times  as  much  of  potas- 
sium as  of  aluminium  ion.  The  ab- 
scissas, Fig.  10,  represent  the  con-  | 
centration  in  the  solution  necessary  | 
for  precipitation.  These  relations  r 
have  been  almost  quantitatively 
confirmed  by  Freundlich. 

As  an  example  of  how  this  law 
may  be  made  use  of,  an  article  by 
Galecki  may  be  cited.  Many  inves- 
tigators have  doubted  the  valence 
of  beryllium.  Galecki  *  determined  the  precipitation  values  of  differ- 
ent salts  on  As2Sa5.  Table  10  gives  the  results  obtained.  It  can  be 
seen  that  beryllium  comes  in  the  same  category  as  the  bivalent  metals. 

TABLE   10 

20  cc.  AsoSsS  76  mille-mols.  per  liter 


. 

BSf 

t* 

S1 


Concentration  of  the  solution 

FIG.  10. 


0  01N  Ba  (NO3)2 

1  85 

1  9 

1.85  cm. 

0  01     BeSO4 

1.85 

1.8 

1.85    " 

0  01    Be  (NO3)2                   .... 

1.85 

1.85 

1.8     " 

0  01     La  (NO3)3  

0.15 

0.15    % 

0.15    " 

0.01     A1C13  

0.1 

0.15 

0.15    " 

20  cc.  As-jSsS  150  mille-mols.  per  liter 


0.01NBa(NO3)2.. 

0.9 

0.9 

0.9    cc. 

0  01     Be  (NO3)2 

0  95 

0  9 

0.9     " 

0  01    Mg  (NO3)2 

0.8 

0.9 

0.9     " 

0  01     La  (NO3)3 

0.05 

0.05 

Mutual  Precipitation  of  Colloids 

In  close  association  with  the  coagulation  by  electrolytes  is  the  mutual 
precipitation  of  two  colloids  having  charges  opposite  to  each  other  in 
nature.  Many  investigators  have  worked  with  this  phenomenon,  but 
Biltz  was  the  first  to  give  a  satisfactory  explanation.!  Graham  remarked 
this  mutual  precipitation,  while  Picton  and  Linder  %  made  the  obser- 
vation during  their  investigations  with  solutions  of  dyestuffs,  that 

*  A.  v.  Galecki:  Zeit.  f.  Elektrochemie,  14,  767-768  (1897). 

t  W.  Biltz:  Her.,  37,  1095-1116  (1904). 

j  H.  Picton  and  S.  E.  Linder:  Journ.  Chem.  Soc.,  71,  568-573  (1897). 


56  CHEMISTRY  OF  COLLOIDS 

mixtures  of  those  with  charges  opposite  in  sign  were  mutually  precipi- 
tated. On  the  other  hand  those  with  like  charges  had  no  effect  on  each 
other.  Similar  effects  were  noted  by  Lottermoser  *  with  colloidal  iron 
oxide,  silicic  acid,  and  several  other  colloids.  He  endeavored  to  deter- 
mine the  composition  of  the .  precipitate,  but  the  experiments  were 
frustrated  by  difficulties  in  washing  it,  owing  to  its  colloidal  nature. 

Contrary  to  the  experiences  of  Picton  and  Linder,  certain  observa- 
tions of  Spring  f  seemed  to  upset  all  the  rules  made  up  to  this  time  for 
the  mutual  precipitation  of  colloids.  It  remained  for  the  excellent 
work  of  Biltz,  however,  to  elucidate  the  relations  and  give  us  a  very 
important  orderly  arrangement  in  this  part  of  colloidal  chemistry. 
According  to  the  law  that  obtains  in  these  cases,  colloids  having  oppo- 
site charges  always  precipitate  each  other  if  they  are  mixed  in  the 
proper  proportions.  On  the  contrary  if  either  one  is  present  in  an 
appreciable  excess  there  will  be  no  coagulation.  Herein  lies  the  cause 
of  Spring's  anomalous  results;  the  mixture  was  not  made  in  the  requi- 
site proportions.  About  the  same  time  Bechhold,t  Neisser  and  Fried- 
mann,§  Victor  Henri  ]f  and  his  fellow  workers,  all  obtained  results 
similar  to  Biltz.  Some  of  Biltz's  results  are  given  in  Table  11.  This 

TABLE  11 

Sb2S3-sol.  against  Fe2O3-sol. 
2  cc.  of  the  sulfide  solution  =  0.56  mgs.  SbaSs. 
13  cc.  iron  oxide  solution  of  variable  concentration. 


mg.  Fe2Oa 


20.8 

12.8 

8.0 

6.4 
4.8 
3.2 
0.8 


Observations  made  immediately  after  mixing. 


Turbid,  but  homogeneous. 
Turbid,  but  homogeneous. 

Flocculent  precipitate  slowly  settled  out.     Solu- 
tion yellow. 

Complete  precipitation. 
Formation  of  flocks,  solution  yellow. 
Few  flocks,  solution  yellow. 
Turbid,  no  flocks,  solution  yellow. 


table  shows  that  to  obtain  a  maximum  precipitating  effect  a  definite 
relative  concentration  must  be  maintained,  and  that  if  this  relation  is 
exceeded  too  far  in  either  direction  there  will  be  no  coagulation.  In 

*  A.  Lottermoser:  Anorganische  Kolloide,  page  76.     Stuttgart  (1901). 

t  W.  Spring:   Bulletin  de  1'Acad.  Roy.  de  Belg.  (3),  38,  483-520  (1900). 

J  H.  Bechhold:  Zeit.  f.  phys.  Chemie,  48,  385-423  (1904). 

§  M.  Neisser  und  U.  Friedmann:  Munch,  med.  Wochenschr.,  51,  465-469,  827- 
831  (1903-4). 

TI  V.  Henri,  Lalou,  Mayer  et  Stodel:  Comp.  rend,  des  stances  de  la  Soc.  de  Biol., 
66,  1666  (1904). 


PROPERTIES  OF  COLLOIDS  57 

this  respect  mixtures  of  colloids  differ  from  those  of  electrolytes. 
Barium  chloride  and  Glauber  salts  always  give  a  precipitate  regardless 
of  the  relative  amounts  added.  The  amount  of  the  residue  is  regu- 
lated by  the  substance  present  in  lesser  quantity. 

The  precipitation  of  the  mixture  only  within  certain  fixed  limits  of 
concentration  is  explained  by  the  fact  that  the  electric  charges  on  the 
particles  must  be  completely  neutralized  before  the  maximum  coagula- 
tion can  be  obtained.  If  one  or  other  of  the  two  colloids  is  present 
in  excess,  the  oppositely  charged  particles  will  unite,  and  only  those 
unions  that  are  exactly  neutral  will  be  precipitated.  Finally  when  one 
constituent  is  present  in  large  excess  there  is  no  neutralization  of  the 
ultramicrons  because  the  substance  in  the  smaller  quantity  is  com- 
pletely adsorbed  by  the  one  present  in  excess.  The  aggregates  thus 
formed  will  have  the  same  charge  as  the  colloid  present  in  the  larger 
amount.  Similar  relations  are  met  with  sometimes  when,  at  great 
dilution,  very  insoluble  precipitates  are  formed  in  solutions  of  crystal- 
loids. The  essential  conditions  for  the  formation  of  a  sol  by  precipita- 
tion reactions  are  that  one  ion  should  be  adsorbed  to  a  greater  degree 
than  the  other.  See  Chapter  V  on  colloidal  silver. 

Biltz  *  found  further  that  where  acid  and  basic  dyestuffs  had  oppo- 
site charges  and  were  mutually  precipitated,  the  formation  of  an  in- 
soluble chemical  combination  could  be  assumed.  This  corresponds  to 
what  is  known  of  acid  and  basic  dyestuffs  in  the  state  of  crystalloids; 
"namely  that  they  unite,  and  when  the  combination  is  insoluble  a  pre- 
cipitate is  of  course  formed.  Doubtless  such  reactions  occur  in  the 
case  of  many  dyestuffs.  The  more  colloidal  the  character  of  these 
dyestuffs  the  more  nearly  will  the  laws  of  colloidal  reaction  obtain. 
Moreover  Biltz  has  pointed  out  that  with  colloids  of  the  nature  of  gold 
such  chemical  reactions  are  not  possible.  Nevertheless  colloidal  gold 
is  precipitated  by  positively  charged  dyestuffs.  The  cause  is  doubt- 
less the  neutralization  of  the  charge  on  the  particles.  For  further 
particulars  attention  is  called  to  Chapter  XL 

Absorption  and  Adsorption 

It  is  very  well  known  that  comparatively  large  quantities  of  carbon 
dioxide  and  ammonia  are  condensed  by  porous  substances,  such  as 
charcoal,  meerschaum,  etc.,  although  no  chemical  reaction  occurs 
between  them  at  the  temperature  in  question.  In  fact  charcoal  will 
not  only  take  up  gases,  but  even  dyestuffs  and  salts  of  the  heavy  metals 
can  be  removed  from  a  solution  by  this  means.  Use  is  made  in  the 
industries  of  this  property  to  clarify  sugar,  to  remove  fusel  oil  from 
*  W.  Biltz:  Ber.,  37,  1111  (1904). 


58  CHEMISTRY  OF  COLLOIDS 

alcohol  and  alcoholic  beverages,  and  to  take  bad  smelling  substances 
from  gas  mixtures. 

The  taking  up  of  gases  and  dissolved  substances  by  charcoal  was 
looked  upon  as  a  reaction  analogous  to  the  dissolution  of  a  gas  by 
liquid,  and  was  therefore  given  the  name,  absorption.  The  character- 
istics of  the  phenomenon  were  that  the  gas  penetrated  into  the  interior 
of  the  porous  body.  Later  a  distinction  was  made  between  ab- 
sorption and  adsorption.  The  former  was  employed  if  the  gas  pene- 
trated into  the  body;  the  latter  when  the  gas  was  condensed  on  the 
surface.  (Mueller  and  Pouillet.*)  From  this  point  of  view  van  Bem- 
melen  designated  the  taking  up  of  gases  and  dissolved  substances  by 
hydrosols  as  absorption  and  not  adsorption.  The  nomenclature  was 
justifiable  at  the  time,  because  there  can  be  no  question  but  that  the 
substance  taken  up  penetrates  the  entire  mass,  and  does  not  remain 
on  the  outer  surface  as  does  gas  on  pieces  of  quartz  or  glass. 

Since  porous  substances  and  hydrogels  have  come  to  be  regarded  as 
heterogeneous  systems  having  numerous  walls  or  partitions  throughout 
the  mass,  the  term  adsorption  has  again  been  used,  with  the  corre- 
sponding limitations  on  the  term,  absorption.  It  is  open  to  question, 
however,  whether  the  taking  up  of  gases  and  dissolved  substances  by 
hydrogels  is  solely  a  surface  phenomenon.  Van  Bemmelen  and  others 
have  argued  against  this  point  of  view.  Details  cannot  be  dealt  with 
here;  a  fundamental  principle,  however,  will  be  elucidated. 

Imagine  two  bodies,  each  homogeneous  in  itself,  touching  each  other 
but  sharply  differentiated  at  the  point  of  contact,  such  as  a  quartz 
crystal  or  glass  and  air.  That  is  to  say,  there  shall  be  no  solution  of 
one  in  the  other.  Their  common  border  is  a  true  surface  in  the  geo- 
metric sense.  It  is  clear  that  on  such  a  two-dimensional  surface  no 
substance  occupying  three  dimensions  can  accumulate.  When,  how- 
ever, a  gas  condenses  on  the  surface  of  a  solid  body  it  must  either 


on  n  noon 


ooo  oooo 


Pure  Adsorption. 

FIG.  11.    Penetration  of  gases,  etc.,  through  the  outer  surface. 

accumulate  in  a  layer  on  the  outer  surface,  or  penetrate  the  latter  to  a 
certain  distance.  Both  effects  are  illustrated  in  Fig.  11.  Examples  of 
both  these  phenomena  are  known.  According  to  Weidele  f  the  well- 

*  Miiller-Pouillet:  Lehrbuch  der  Physik  (9.  Aufl.),  1,  589.     Braunschweig  (1886). 
t  Ibid. 


PROPERTIES  OF  COLLOIDS  59 

known  frost  figures  on  the  window  pane  arise  from  the  gas  that  has  been 
condensed  on  the  surface.  On  the  other  hand,  the  cracks  and  bubbles 
that  occur  when  an  old  piece  of  glass  is  being  heated  are  due  to  water 
and  carbon  dioxide  that  have  penetrated  the  glass. 

Only  in  the  case  where  the  condensation  occurs  on  the  outer  surface 
is  the  term  adsorption  appropriate.  Although  water  vapor  and  carbon 
dioxide  reach  the  interior  of  many  bodies,  the  penetration  of  crystals  is 
confined  to  a  very  thin  layer  while  the  composition  of  the  interior  re- 
mains unchanged.  Quite  different  will  be  the  conditions  when,  a  solid 
is  in  thin  layers  or  lamellae.  If  the  lamellae  are  thinner  than  the  dis- 
tance to  which  the  gas  will  penetrate,  the  composition  of  the  entire 
mass  will,  of  course,  change.  As  a  consequence  the  properties  of  a  sub- 
stance in  the  form  of  lamellae  may  be  quite  different  from  those  of  the 
same  body  in  a  more  compact  state.  In  the  latter  case  the  phenome- 
non is  purely  one  of  adsorption,  while  the  lamellae  either  absorb  the 
gas,  or  a  combination  of  both  adsorption  and  absorption  may  occur. 
Colloidal  particles  of  hydrogels  may  be  regarded  as  extraordinarily  fine 
lamellae,  and  although  we  cannot  decide  definitely  whether  the  conden- 
sation is  purely  a  surface  phenomenon  or  a  penetration  (probably  both), 
it  seems  justifiable  with  van  Bemmelen  to  designate  the  taking  up  of 
gases  and  dissolved  substances  by  hydrogels  as  absorption.  Doubt- 
less the  case  is  very  complicated,  for  even  chemical  reactions  may  occur. 
From  these  considerations  it  would  perhaps  be  better  to  drop  both 
terms  in  doubtful  cases  and  employ  the  expression,  sorption,  as  has 
been  suggested  by  McBain.*  It  seems  doubtful,  however,  whether  it 
would  be  practicable  to  change  the  nomenclature  of  so  many  authors. 
Especially  is  this  so  because  of  the  fact  that  good  grounds  exist  for 
believing  that  the  taking  up  of  dissolved  substances  by  hydrogels  is 
in  reality  chiefly  a  matter  of  surface  phenomena. 

It  is  worthy  of  note  that  the  accumulation  of  dissolved  substances  on 
the  surface  of  other  bodies  is  very  general.  Gibbs  f  has  shown  that  it 
must  come  to  pass  whenever  the  dissolved  substance  lowers  the  surface 
tension.  A  great  many  cases  are  known  where  there  is  neither  chemical 
reaction  between  the  adsorbing  and  the  dissolved  substances;  nor  does 
the  latter  penetrate  the  former.  Moreover,  because  hydrogels  doubt- 
less contain  a  large  number  of  inner  surfaces  or  partitions,  true  adsorp- 
tion must  play  an  important  part.  This  is  also  true  of  hydrosols  where 
the  total  surface  is  enormous. 

As  might  be  expected  the  accumulation  on  the  surface  takes  place 

*  J.  W.  McBain:  Zeit.  f.  phys.  Chem.,  68,  471-497  (1908). 
f  J.  W.  Gibbs:    Thermodynamische  Studien.     German  translation  by  W.  Ost- 
wald.     Leipzig  (1892). 


60  CHEMISTRY  OF  COLLOIDS 

much  more  rapidly  than  the  penetration  into  the  interior.  Freund- 
lich  *  has  demonstrated  that  adsorption  equilibria  are  reached  very 
quickly,  while  many  cases  occur  where  equilibrium  is  difficult  to  obtain. 
He  has  assumed  that  the  delay  is  caused  either  by  chemical  reactions 
or  by  the  penetration  of  the  dissolved  substance  into  the  interior  of  the 
solid. 

The  term  adsorption  will  be  used  in  this  book  with  the  understanding 
that  all  the  phenomena  described  under  this  head  are  not  necessarily 
surface  reactions.  The  literature  on  this  subject  is  so  vast  that  none 
but  the  most  salient  points  can  be  taken  up  here.  For  comprehensive 
discussions  reference  should  be  made  to  W.  Ostwald,f  Wo.  Ostwald,!  and 
to  Freundlich.§ 

In  order  to  obtain  a  comprehensive  view  of  the  numerous  facts  in- 
volved some  classification  is  necessary.  The  following  one  has  been 
adopted  for  the  present  purpose. 

1.  Adsorption  of  dissolved  crystalloidal  substances  by  bodies  having 
smooth  surfaces,  and  by  porous  or  powdered  substances. 

2.  Adsorption   of  dissolved   crystalloidal  substances  by  hydrogels 
whereby : 

(a)  The  adsorbed  material  accumulates,  but  the  gel  remains  practi- 
cally unaltered  in  its  properties. 

(6)  The  composition  of  the  gel  is  changed,  and  the  new  substance 
itself  becomes  a  hydrosol.  (Chemical  reactions  are  generally  involved.) 

3.  The  adsorption  of  dissolved  crystalloidal  substances  by  the  ultra- 
microns  of  colloidal  solutions.     Thereby: 

(a)  The  system  may  remain  apparently  unchanged. 

(b)  Precipitation  (coagulation)  of  the  ultramicrons  may  occur,  fol- 
lowing the  neutralization  of  the  charges  on  the  particles  by  ions  of 
opposite  sign.     Doubtless   chemical  reactions  frequently  take   place 
here  also. 

4.  Adsorption  of  colloidal  substances  by  bodies  having  distinct  sur- 
faces, such  as  powders,  charcoal. 

5.  Mutual  adsorption  of  ultramicrons.     For  instance,  protective  col- 
loidal effects. 

Other  classifications  are  possible.  Freundlich,  for  example,  consid- 
ers the  accumulation  of  substances  on  the  surface  under  the  headings: 
Solid-gas,  solid-liquid,  liquid-liquid,  liquid-gas. 

*  H.  Freundlich:  Zeit.  f.  phys.  Chemie,  67,  385-470  (1907). 
t  W.  Ostwald:  Lehr.  d.  allgem.  Chemie  (1  Aufl.),  1,  778-791  (1885);    (2  Aufl.) 
2,  3,  217  ff.  (1906). 

$  Wo.  Ostwald:  Grundriss  der  Kolloidchemie,  390-445.     Dresden  (1909). 
§  H.  Freundlich:  Kapillarchemie.     Leipzig  (1909). 


PROPERTIES  OF  COLLOIDS  61 

i.  Adsorption   of  Dissolved   Crystalloidal   Substances.  —  As   has 

already  been  stated,  Gibbs,  from  theoretical  considerations,  arrived 
at  the  conclusion  that  substances  tending  to  lower  the  surface 
tension  of  a  solution  against  another  phase,  must  be  adsorbed  by  this 
solution.  Further,  small  amounts  of  the  dissolved  substance  may 
lower  the  surface  tension  a  great  deal,  but  cannot  raise  it  very  much.* 
These  laws  have  been  qualitatively  proved  in  many  instances.  They 
obtain  for  crystalloids  as  well  as  for  colloids.  The  investigations  must 
be  confined  to  liquid-gas  or  liquid-liquid  systems,  for  it  is  only  in  these 
cases  that  the  surface  tension  can  be  determined  with  accuracy. 
Freundlich  has  shown  that  those  substances  which  greatly  lower  the 
surface  tension  of  liquid  against  liquid  are  not  only  adsorbed  by  these, 
but  are  also  strongly  adsorbed  by  solids.  Moreover,  he  has  demon- 
strated that  adsorption  isotherms  (which  give  the  relations  quantita- 
tively) apply  to  both  the  surface  liquid-gas  and  liquid-liquid. 

W.  Ostwald  f  showed  that  adsorption  is  reversible  and  that  the  equi- 
librium could  be  reached  from  both  sides.  A  large  number  of  deter- 
minations by  various  investigators  have  shown  that  the  quantitative 
adsorption  of  dissolved  crystalloidal  substances  in  relation  to  the  end 
concentration  can  be  expressed  by  Freundlich's  purely  empirical  formula 

—  =  a  •  cn, 
m 

where  x  is  the  amount  adsorbed,  m  is  the  amount  of  adsorbing  sub- 
stance, c  is  the  end  concentration,  and  a  and  -  are  constants  that  depend 

upon  the  nature  of  the  substances  in  question. 

Other  formulas  are  in  use.  The  following  one  of  G.  C.  Schmidt  t 
holds  very  well  for  the  adsorption  of  acetic  acid  by  charcoal. 

A  (S  -  x) 

=  K-e      s      -x, 


where  a  is  the  original  amount  of  dissolved  substance,  x  is  the  amount 
adsorbed,  v  is  the  volume,  S  is  the  maximum  adsorption,  and  A  and  K  are 
constants.  Svannte  Arrhenius  §  has  recently  evolved  another  equa- 
tion from  theoretical  grounds. 

T~dx      (s  —  x)          .  ,  ,    ,         s          n  ArtA~x      1 

K   —-  °r  '^^d  lo—  -  °-4343  =  c- 


*  J.  W.  Gibbs.     Thermodynamische  Studien,  321. 

t  W.  Ostwald:  Lehrb.  d.  allg.  Chemie,  789  (1  Aufl.). 

t  G.  C.  Schmidt:  Zeit  f.  phys.  Chemie,  77,  641-660  (1911).  R.  Marc:  Zeit.  f. 
phys.  Chemie,  76,  58-66  (1911). 

§  Svante  Arrhenius:  Meddelanden  fran.  K.  Vetenskapsakad.  Nobelinstitut,  2, 
No.  7  (1911).  G.  C.  Schmidt:  Zeit.  f.  phys.  Chemie,  78,  667-681  (1912). 


62  CHEMISTRY  OF  COLLOIDS. 

Here  x  is  the  amount  of  adsorbed  substance  on  1  gm.  of  charcoal,  s  is 
the  maximum  value  of  x,  c  is  the  pressure  of  the  gas  or  the  osmotic 
pressure  of  the  dissolved  substance,  and  k  is  a  constant. 

Attention  is  called  to  the  fact  that,  according  to  van  Bemmelen,* 
the  isotherms  already  mentioned,  within  certain  limits,  apply  to  the 
taking  up  of  crystalloids  by  charcoal  as  well  as  by  hydrogels. 

The  value  of  -  according  to  Freundlich  varies  from  0.1  to  0.5.     The 
n 

values  of  a  vary  within  wide  limits.     For  example  for  the  system 

Acetic  acid  in  water  and  charcoal  from  blood a  =    2.6 

Bromine  in  water  and  charcoal  from  blood a  =  23 . 1 

For  specific  information  in  this  field  the  original  work  of  Freundlich 
should  be  consulted.  It  should  not  be  overlooked,  however,  that  the 
adsorption  isotherms  alrea$y  given  are  typical  for  true  adsorption 
phenomena;  and  wherever  these  isotherms  fit  the  case  many  experi- 
menters regard  the  reactions  as  pure  adsorptions,  even  when  there  is 
some  evidence  to  the  contrary.  It  should  also  be  remembered  that  the 
formula  has  no  theoretical  foundation,  and  that  the  incorporation  of 
two  constants  makes  it  a  very  flexible  interpolation  formula. 

The  theoretically  grounded  formula  of  Arrhenius  containing  one 
constant  assumes  the  adsorption  to  be  due  to  molecular  attraction 
between  the  gas  or  dissolved  substance  and  the  adsorbing  material. 
It  is  in  close  association  with  the  compressibility  of  the  liquids  and  is 
not  connected  with  capillarity. 

2.  Adsorption  of  Dissolved  Crystalloidal  Substances  by  the  Ultra- 
microns  of  Hydrogels.  —  The  taking  up  of  dissolved  crystalloidal  sub- 
stances, during  which  a  spontaneous  subdivision  of  the  hydrogel  occurs, 
will  be  discussed  with  the  theory  of  peptisation  in  Chapter  IV. 

3.  Adsorption  of  Crystalloidal  Substances  by  the  Ultramicrons  of 
Colloidal  Solutions.f  —  An  example  of  this  has  already  been  considered 
on  page  38. 

4.  Adsorption  of  Colloids  by  Other  Substances.  —  Cases  where  a 
colloid  can  be  completely  removed  from  a  liquid  by  porous  bodies, 
powder,  or  jelly-like  substances,  are  of  common  occurrence. %    Colloidal 
gold  can  be  separated  from  the  liquid  by  animal   charcoal,  barium 
sulfate,  or  aluminium  hydroxide  gels.     The  process  may  be  followed 

*  J.  M.  van  Bemmelen:  Journ.  f.  prakt.  Chemie  (2),  23,  324-349,  379-395  (1881); 
Zeit.  anorg.  Chemie,  23,  111-125,  321-372  U903). 

t  A.  Lottermoser  und  P.  Mama:  Ber.  43,  3613-3618  (1910).  Wo.  Ostwald:  von 
Bemmelen-Gedenkboek,  246-274  (1910). 

t  R.  Zsigmondy:  Verh.  d.  Ges.  D.  Naturf.  u.  Artze,  73,  Vers.,  168-172.  Hamburg, 
(1901).  L.  Vanino:  Ber.,  35,  662  (1902).  W.  Biltz:  Nachr.  d.  Kgl-Ges.  d.  Wiss. 
Gottingen,  Math.  phys.  Kl.,  46-63  (1905). 


PROPERTIES  OF  COLLOIDS  63 

partially  at  least  with  the  ultramicroscope.  It  is  improbable  that 
electric  charges  play  a  part  here  because  animal  charcoal  and  colloidal 
gold  are  both  charged  negatively.  It  is  much  simpler  to  assume  some 
specific  attraction  between  the  adsorbing  substance  and  the  gold. 

Many  colloids  other  than  gold  can  be  removed  from  solution  by  the 
same  method.  As  a  lecture  experiment  the  author  uses  the  adsorption 
of  molybdenum  blue  by  animal  charcoal.  The  dark  blue  liquid  becomes 
quite  colorless  on  shaking  and  filtering.  As  in  many  other  cases  the 
adsorption  is  quantitative  and  irreversible. 

It  is  assumed  by  many  investigators  that  during  adsorption  only  that 
part  of  the  substance  present  as  crystalloid  is  taken  up.  This  is  prob- 
ably true  in  many  cases;  as,  for  instance,  in  coloring  with  dyestuffs 
where  the  portion  present  as  crystalloid  is  large  and  the  remainder  is  in 
the  form  of  relatively  large  particles.  This  point  of  view  is  untenable, 
however,  in  the  case  of  true  colloids  where  the  part  dissolved  as  crystal- 
loid is  very  small.  Here  it  must  be  assumed  that  the  colloidal  particles 
are  themselves  adsorbed.  In  specific  instances  this  can  be  verified 
experimentally  as  in  the  case  of  gold  and  aluminium  hydroxide  gels. 

Mutual  Adsorption  of  Ultramicrons.  Protection  Effects. — The 
adsorption  of  colloids  by  finely  divided  charcoal  is  closely  allied  to  the 
taking  up  of  colloids  by  the  ultramicrons  of  other  colloids.  It  plays 
as  important  a  role  in  the  precipitation  of  colloids  as  it  does  in 
protective  effects.  In  general  the  protective  colloid  is  taken  up  or 
adsorbed  by  the  particles  of  the  irreversible  colloid,  even  though  the 
signs  of  the  electric  charges  are  the  same.  Occasionally  the  process  is 
reversed,  and  the  particles  of  the  irreversible  are  adsorbed  by  the  pro- 
tective colloid.  This  last  phenomenon  may  be  followed  under  the 
ultramicroscope  when  the  protective  colloid  is  present  in  a  not  too  fine 
state  of  division.  See  Chapter  V. 

The  cause  of  the  protective  effect  is  a  union  between  the  particles  of 
different  colloids.  Ultramicrons  of  an  irreversible  colloid,  that  have 
taken  up  a  sufficient  number  of  particles  of  a  protective  colloid,  have 
lost  the  property  of  coagulation  either  on  the  addition  of  an  electrolyte, 
or  on  evaporation.  The  protective  effect  of  specific  colloids  varies 
greatly.  A  basis  for  comparison  may  be  found  in  the  determination  of 
the  gold  number.  This  together  with  other  points  of  interest  will  be 
discussed  in  Chapter  V. 

Influence  of  Temperature  on  Colloids 

At  higher  temperatures  colloids  are  in  general  less  stable  than  at 
lower.  The  specific  members,  however,  differ  very  greatly  in  this  re- 
gard. As  an  instance  of  this  colloidal  gold  solutions  may  be  cited.  If 


64  CHEMISTRY  OF  COLLOIDS 

made  according  to  the  author's  procedure  the  solution  will  withstand 
heating  at  the  boiling  temperature  for  a  long  time  without  suffering 
any  change.  On  the  contrary  Faraday's  colloidal  gold  will  coagulate  at 
the  boiling  point,  as  will  also  the  pla.tinum  hydrosols  prepared  accord- 
ing to  Bredig's  method.  Graham's  colloidal  aluminium  hydrate  solu- 
tions could  be  heated  to  the  boiling  temperature  with  impunity;  they 
coagulated,  however,  if  the  boiling  were  continued.  Many  irreversible 
hydrosols  exhibit  a  similar  behavior.  Schneider  *  has  reported  that  col- 
loidal silver  coagulates  completely  if  the  solution  is  heated  in  a  closed 
tube  to  the  critical  temperature.  Protein  solutions  coagulate  consider- 
ably under  the  boiling  temperature,  while  gelatin  in  solution,  on  the 
other  hand,  becomes  more  finely  divided  at  the  higher  temperatures. 

Freezing  of  Colloids.  —  On  freezing  the  disperse  medium,  colloids 
show  a  great  variation  in  behavior.  Many  of  them  suffer  irreversible 
changes  so  that  after  the  thawing  out  of  the  frozen  mass,  they  no 
longer  form  a  colloidal  solution,  but  are  coagulated  into  jellies,  powders, 
or  fine  flakes.  Pure  metal  hydrosols  are  completely  precipitated  and 
the  residue  cannot  be  returned  to  the  colloidal  state.  Protected  metal 
colloids,  on  the  contrary,  may  be  frozen  repeatedly  without  coagula- 
tion occurring.  In  the  case  of  colloidal  sulfides  and  oxides  the  amount 
of  peptising  agent  present  is  of  importance.  In  general  it  may  be  said, 
the  more  stable  a  colloid  is  toward  electrolytes  and  evaporation,  the 
more  stable  it  will  be  to  freezing.  In  this  connection  Ljubavinf  found 
that  colloidal  iron  oxide  suffered  a  change  on  freezing  only  if  it  had  been 
previously  well  dialyzed.  Lottermoser  J  corroborated  this  result  and 
showed  that  the  colloid  would  coagulate  on  freezing  if  it  were  nearly 
free  from  electrolytes.  It  was  also  noticed  that  the  conductivity  of  the 
system  was  increased  by  the  freezing.  This  can  be  accounted  for  on 
the  grounds  that  the  particles  give  up  the  adsorbed  ions,  which  are 
necessary  for  the  stability  of  the  colloid,  and  suffer  an  irreversible 
change.  On  melting  the  mass  the  colloid  has  thus  lost  the  property  of 
peptising.  Gelatin,  sturgeon  bladder,  carrageen,  agar-agar,  and  sapo 
medicatus  change  so  that  the  first  portions  of  the  liquid  after  melting 
the  frozen  mass  contain  scarcely  any  of  the  dissolved  substance.  After 
complete  thawing  out  the  mass  is  inhomogeneous,  consisting  of  thin 
liquid  and  flocculent  jelly.  At  room  temperature  the  process  of  redis- 
solution  is  not  complete  after  48  hours.  § 

*  E.  A.  Schneider:  Zeit.  f.  anorg.  Chemie,  3,  78-79  (1893). 
t  K.  Ljubavin:    Journ.  russ.  phys.  chem.  Ges.,  21,   1,   397-407   (1889).     Ref. 
Koll.-Zeit.,  1,  53  (1906). 

J  A.  Lottermoser:  Zeit.  f.  phys.  Chemie,  60,  462  (1907);  Ber.,  41,  3976-3979  (1908). 
§  O.  Bobertag,  K.  Feist  und  H.  W.  Fischer:  Ber.,  41,  3675-3679  (1908). 


PROPERTIES  OF  COLLOIDS 


65 


FIG.  12. 


During  his  work  on  the  effect  of  low  temperatures  on  plants,  Molisch  * 
investigated  the  changes  in  jellies  brought  about  by  freezing.  Fig.  12 
shows  how  the  gelatin  arranges  itself  in  a 
network  of  cellular  structure  around  the 
tiny  lumps  of  ice.  H.  Ambronn  f  has 
shown  that  the  colloidal  net  of  gelatin  or 
agar-agar  resembles  in  many  respects  a 
parenchymous  plant  tissue.  In  optical 
properties  the  walls  of  the  net  are  very 
similar  to  the  cell  walls  of  a  normal  plant. 
Ambronn  also  demonstrated  that  ice 
flowers  may  be  fixed  upon  a  glass  plate 
if  a  solution  of  gum  is  spread  out  upon 
it,  allowed  to  freeze,  and  the  ice  evapo- 
rated at  low  temperatures.  An  interest- 
ing physico-chemical  phenomenon  was 
discovered  by  H.  W.  Fischer,  t  who  studied  the  rate  of  cooling  of  dif- 
ferent jellies.  He  found  that  the  curve  below  the  freezing  point  did 
not  always  correspond  with  that  of  water. §  From  this  it  was  con- 
cluded that  jellies  undergo  changes  on  freezing  by  which  heat  is 
evolved. 

Similar  changes  of  state  take  place  on  freezing  in  the  case  of  plants 
and  animal  tissues.  The  analogy  to  the  behavior  of  hydrogels  is  quite 
striking.  Both  in  the  case  of  jellies  and  in  that  of  leaves,  fruit,  and 
muscles  after  thawing  the  mass,  the  water  is  not  so  firmly  held  as  be- 
fore the  freezing.  Mere  freezing  does  not  necessarily  destroy  the  hydro- 
sols.  The  temperature^of  the  gelatin  solution  must  be  lowered  to  such  a 
degree  that  irreversible  changes  come  to  pass.  For  the  state  of  the 
water  in  the  muscles  after  freezing,  see  the  article  by  P.  Jensen  and 
H.  W.  Fischer.lf 

Heat  of  Colloidal  Reactions 

The  heat  of  colloidal  reactions  is  generally  very  small.  That  it  is 
appreciable  during  distension  and  dissolution  of  gelatin  has  been  shown 
by  Wiedemann  and  Luedeking.||  The  results  for  irreversible  colloids 

*  H.  Molisch:  Untersuchungen  iiber  das  Erfieren  der  Pflanzen.    Jena  (1897). 

t  H.  Ambronn:  Verb.  d.  Kgl.  sachs.  Ges.  d.  Wiss.  Leipzig  Math.  phys.  KL,  43, 
28-31  (1891). 

J  H.  W.  Fischer:  Beitrage  zur  Biologic  der  Pflanzen,  10,  133-234  (1910). 

§  H.  W.  Foote  and  Blair  Saxton,  Jour.  Am.  Chem.  Soc.,  38,  588  (1916). 

f  P.  Jensen  und  H.  W.  Fischer:  Zeit.  f.  dig.  Physiol,  11,  23-93  (1910). 

||  E.  Wiedemann  und  Ch.  Ludeking:  Wiedemanns  Annalen  (N.  F.),  26,  145-153 
(1885). 


66 


CHEMISTRY  OF  COLLOIDS 


are  often  contradictory.  For  instance,  J.  Thomsen  *  observed  no  heat 
of  reaction  during  the  coagulation  of  silicic  acid,  while  Wiedemann  and 
Luedeking  obtained  11.3  to  12.2  calories  per  gram.  Graham  f  ob- 
served a  rise  in  temperature  of  one  degree  during  the  coagulation  of  a 
5  per  cent  gelatin  solution.  According  to  Picton  and  Linder  {  there  is 
no  measurable  heat  of  reaction  during  the  gelatinizing  of  As2S5,  As2Ss, 
and  Fe203. 

A  very  careful  research  by  Doerinckel  §  has  demonstrated  that  the 
heat  of  reaction  during  the  coagulation  of  silicic  acid  and  also  of  iron 
oxide  is  always  positive.  Its  relation  to  the  concentration  of  the  hydro- 
sol  and  to  that  of  the  precipitating  agent  is  shown  by  the  curves  in 
Figs.  13  and  14.  The  ordinates  give  the  heat  of  reaction  in  calories 


,180 
160 
140 
120 
.100 


40 


Concentration  of  KS  C2  04in  Equivalents 

FIG.  13.  'Heat  of  coagulation  of  a  10.8% 
solution  of  Fe2O3  by  K2C2O4. 


1          2          3 

Concentration  of 

'in  Equivalents 


FIG.  14.     Heat  of  coagulation  of  a  5% 
solution  of  FegOa  by  KsCsO4. 


for  250  cc.  of  the  solution,  while  the  abscissas  give  the  concentration  of 
the  precipitating  reagent  in  equivalents  per  liter.  The  heat  of  coagu- 
lation as  a  function  of  the  amount  of  the  adsorbed  electrolyte  is  shown 
in  Fig.  15.  The  heat  of  coagulation  depends  among  other  things  upon 
the  nature  of  the  precipitating  agent.  The  value  obtained  by  Doer- 
inckel for  the  precipitation  of  iron  oxide  by  potassium  oxalate  is  almost 
three  times  as  great  as  that  by  aluminium  sulfate.  The  investigation 
proved  further  that  the  heat  evolved  is  not  a  linear  function  of  the 
concentration  of  the  colloid.  The  linear  extrapolation  generally  gives 

*  J.  Thomsen:   Thermochemische  Untersuchungen,  1,  211-219.      Leipzig  (1882). 
t  Th.  Graham:  Poggendorffs  Annalen,  123,  529-541  (1864). 
j  H.  Picton  and  S.  E.  Linder:  Jour.  Chem.  Soc.,  61,  144,  146-153  (1892). 
§  F.  Doerinckel:  Zeit.  f.  anorg.  Chemie,  66,  20-36  (1910). 


PROPERTIES  OF  COLLOIDS 


67 


a  value  greater  than  zero.  From  this  it  will  be  seen  that  the  heat  of 
coagulation  is  greater  per  gram  iron  oxide  for  dilute  solutions  than  it  is 
for  the  more  concentrated.  The  cause  of  this  deviation  is  doubtless 
the  fact  that  when  the  more  concentrated  solutions  are  mixed,  mem- 
branes of  the  precipitate  are  formed.  These  membranes  prevent  a 
thorough  mixture  of  the  two  solutions,  and  there  is  not  a  complete 
precipitation  of  the  iron  oxide  during  the  time  of  the  observation. 

The  heat  of  coagulation  is  also  dependent  upon  the  amount  of  pep- 
tising  electrolyte  present. 

Very  interesting  also  is  the  experimental  result  obtained  by  Doer- 
inckel  *  in  regard  to  the  relation  of  the  concentration  of  the  two  com- 
ponents in  the  mutual  precipitation  of  colloidal  silver  (Argoferment, 


I'lO 
160 
150 
HO 
130 
120 
110 
100 
on 

>r 

/ 

/ 

/ 

i 

/ 

t 

/ 

? 

_j—  -•  - 

^-"* 

-^ 

~~^ 

0.20    0.21     0.22     0.23     0.24     0.25     0.26     0.27    0.38     0.29      g 
Adsorbed  K2C2O4  ing 

FIG.  15.    Heat  of  coagulation  of  250  cc.  Fe2O3  solution  as  function 
of  adsorbed  K2C2O4. 

Heyden,  Radqbeul)  and  colloidal  iron  oxide.  As  much  as  47  cals.  for 
every  2.5  gms.  of  the  disperse  phase  were  evolved.  The  maximum  heat 
of  reaction  and  the  precipitation  did  not  correspond.  The  argoferment 
contained  an  appreciable  amount  of  protective  colloid  that  did  not  take 
part  in  the  reaction.  In  the  case  of  the  reversible  precipitation  of 
argoferment  with  ammonium  nitrate  a  very  small  heat  of  reaction  was 
observed,  namely  between  1  and  2  cals.  for  every  gram  of  silver. 
Prange,t  on  the  other  hand,  working  with  pure  colloidal  silver  found 
126.7  and  250.9  cals.  for  1  gm.  silver.  From  this  it  would  appear  that 
the  preparation  of  the  sample  plays  an  important  part,  after  allowance 
is  made  for  the  inaccuracy  of  Prange's  results.  These  results  explain 
the  contradictory  observations  of  different  investigators.  Different 

*  F.  Doerinckel:  Zeit.  f.  anorg.  Chemie,  67,  161-166  (1910). 

t  J.  A.  Prange:  Recueil  d.  travaux  chim.  des  Pays-Bae,  9,  121-133  (1890). 


68  CHEMISTRY  OF  COLLOIDS 

heats  of  coagulation  may  be  obtained  according  to  the  method  of  pre- 
paring the  colloidal  solution,  the  amount  of  electrolyte  present,  the 
fineness  of  the  subdivision,  and  the  nature  and  the  concentration  of  the 
precipitating  reagent.  Attention  should  be  called  to  the  apparatus 
employed  by  Prange  during  these  measurements  for  which  reference 
must  be  made  to  the  original  article. 

Constitution  of  Jellies 

Jellies  may  be  obtained  from  colloidal  solutions  as  well  as  from  col- 
loidal solids  by  a  large  number  of  methods.  They  may  also  differ  mark- 
edly in  their  properties  and  their  structure.  In  fact  it  is  difficult  to 
make  general  statements  about  the  matter.  On  the  one  hand  the 
hydrogel  of  silicic  acid  is  obtained  by  the  coagulation  of  the  hydrosol, 
and  neither  by  dilution  with  water  nor  by  warming  can  the  colloidal 
solution  be  reformed.  On  dehydration  a  glassy  porous  solid  is  left, 
which  will  be  discussed  in  a  later  chapter.  On  the  other  hand  gelatin 
jellies  are  made  by  the  distension  of  solid  gelatin  in  water,  or  by  cooling 
the  solution  which  was  obtained  by  the  aid  of  heat.  The  gel  becomes 
a  liquid  on  warming  and  returns  again  on  cooling.  On  evaporation 
a  transparent  solid  remains  that  will  again  distend  in  water  and  become 
a  hydrosol.  Agar-agar  and  starch  behave  similarly.  Coagulated  al- 
bumin is  an  opaque  cloudy  gel  that  shrivels  to  a  transparent  mass. 
This  mass  will  distend  in  water  to  form  a  hydrosol. 

Concerning  the  structure  of  gels  a  great  many  investigators  have 
occupied  themselves  without  coming  to  any  very  definite  understanding. 
Biologists  as  well  as  chemists  and  physicists  have  been  engaged  in  this 
work.  The  oldest  theory  assumed  a  porous  structure  for  distensible 
bodies.  The  water  penetrated  into  the  pores  and  caused  them  to  swell. 
The  water,  therefore,  was  considered  as  being  held  by  capillary  or  by 
molecular  attraction.  In  1858  Nageli  *  called  attention  to  the  very 
great  difference  between  porous  bodies,  on  the  one  hand,  and  gels  on  the 
other.  He  proposed  a  theory  that  with  some  modifications  explains, 
according  to  the  view  of  the  author,  the  structure  of  gels.  Nageli  pic- 
tured distensible  bodies  as  small  anistropic,  crystal-like  molecular  com- 
plexes or  tiny  crystals,  that  cause  the  double  refraction  because  of  their 
orientation.  According  to  this  theory  the  distension  is  occasioned  by 
the  penetration  of  water  into  the  micellular  walls  in  such  a  manner  that 
the  micells  are  surrounded  by  a  layer  of  water.  The  thickness  of  the 
sheath  is  regulated  by  the  fact  that  the  attraction  of  the  micells  for 
the  water  molecules  diminishes  faster  with  the  distance  than  does 

*  C.  v.  Nageli  und  S.  Schwendener :  Das  Mikroskop  (2.  Auff.).  Leipzig  (1877). 
C.  v.  Nageli:  Theorie  der  Garung,  121  ff.  Munchen  (1879). 


PROPERTIES  OF  COLLOIDS  69 

the  attraction  of  the  micells  for  one  another.*  When  dissolution  of 
the  colloid  takes  place  it  does  not  divide  up  into  individual  molecules, 
but  rather  into  these  molecular  complexes  or  micells.  As  a  matter  of 
fact  we  know  that  the  presence  of  ultramicrons  is  one  of  the  most 
common  occurrences,  and  in  some  cases  a  lamellar,  anisodiametrical 
formation  of  these  ultramicrons  has  been  observed.  We  also  know  that 
the  particles  contained  in  hydrosols  are  not  the  molecules  of  the  sub- 
stance in  question,  but  are  larger  complexes. 

Biitschli  f  carried  out  some  very  thorough  experimental  investiga- 
tions on  the  structure  of  distended  substances.  He  found  that  many 
jellies  possessed  a  fine  honeycomb  micro-structure,  and  sought  for  evi- 
dence to  show  that  jellies  homogeneous  to  the  microscope  were  also  of 
a  honeycomb  formation.  In  order  to  render  the  structure  visible 
Biitschli  employed  many  devices,  such  as  the  hardening  of  gelatin  gels 
by  alcohol  or  chromic  acid.  By  this  treatment  the  jellies  become  tur- 
bid and  more  suitable  for  microscopical  investigation.  He  began  with 
a  research  on  the  structure  of  foams,  obtained  by  the  distension  of  oils 
containing  soap  in  water.f  In  such  a  system  a  honeycomb  structure  is 
formed,  the  walls  of  which  are  oil  lamellae  while  the  enclosed  spaces  are 
filled  with  water.  Similarly  distended  jellies  are  heterogeneous.  The 
small  cells  are  filled  with  water  while  the  extremely  thin  walls  are 
composed  either  of  the  distended  substance,  a  combination  of  it  with 
the  water,  or  a  solid  solution  of  water  in  the  substance.  The  walls 
Biitschli  considers  permeable  to  liquids  because  of  their  extreme  thin- 
ness. Thus  a  diffusion  reaction  may  occur  between  the  enclosed  water 
and  another  liquid,  such  as  alcohol.  He  admits,  however,  that  the 
walls  may  also  be  porous  but  that  these  pores  cannot  be  seen  by  the 
microscope  and  must  therefore  be  very  small. 

Wolgang  Pauli  §  has  raised  an  objection  to  the  assumption  that  the 
structure  of  gels  was  not  altered  by  the  hardening  process  with  alcohol, 
or  with  chromic  acid.  His  view  has  been  substantiated  somewhat  by 
experimental  results  obtained  in  the  laboratory  of  the  author.  The 
ultramicroscopy  of  gelatin  gels,  as  well  as  of  semi-liquid  hydrosols  of. 
silicic  acid,  has  shown  that  the  structure  of  gels  is  granular  or  floccu- 
lent  rather  than  honeycombed.  The  observation  of  the  formation  of  a 

*  Nageli:  Theorie  der  Garung,  148. 

t  0.  Biitschli:  Uber  den  Ban  quellbarer  Korper  usw.  Gottingen  (1896).  Unter- 
suchungen  iiber  Strukturen.  Leipzig  (1898).  Untersuchungen  uber  die  Mikro 
struktur  kiinstlicher  und  natiirlicher  Kieselsauregallerten.  Heidelberg  (1900)  u.  a. 

J  0.  Biitschli:  Untersuchungen  iiber  mikroskopische  Schaume  und  das  Proto- 
plasma.  Leipzig  (1892). 

§  W.  Pauli:  Der  kolloidale  Zustand  und  die  Vorgange  in  der  lebendigen  Sub- 
stanz.  Braunschweig  (1902). 


70  CHEMISTRY  OF  COLLOIDS 

gel  from  submicrons  in  motion  also  substantiates  this  view.  See  Chap- 
ter XII. 

Careful  investigation  of  the  hydrogel  of  dried  silicic  acid  has  shown 
that  the  structure  is  much  finer  than  Butschli  assumed.  As  a  matter 
of  fact  we  are  dealing  with  an  almost  homogeneous  mixture  of  air  and 
silicic  acid;  that  is  to  say,  a  conglomerate  of  silicic  acid  amicrons  per- 
meated with  amicroscopic  empty  spaces. 

The  results  that  have  been  obtained  up  to  the  present  time  with  the 
ultramicroscope  are  far  more  favorable  to  Nageli's  theory  of  distensible 
bodies,  than  to  that  of  Butschli.  As  already  pointed  out  the  pheno- 
mena of  distention  is  not  so  simple  as  Nageli  assumed  in  that  a  larger 
heterogeneity,  discovered  by  Butschli,  is  often  superimposed  upon  a 
finer  discontinuity. 

Weimarn  *  has  pointed  out  that  gels  may  be  formed  simultaneously 
with  crystalline  precipitates  from  solutions  of  high  concentration,  where 
only  crystals  will  be  obtained  if  the  concentration  is  not  so  great. 

*  P.  P.  von  Weimarn:  Zur  Lehre  von  den  Zustanden  der  Materie  Koll.-Zeit.,  2-5 
(1907-1909). 


CHAPTER  IV 
THEORY 

THE  discussion  that  follows  is  not  by  any  means  a  summary  of  the 
theories  that  have  appeared  on  colloidal  chemistry  up  to  present  time. 
Such  a  comprehensive  presentation  would  be  entirely  beyond  the  scope 
of  this  book.*  On  the  contrary  the  author,  with  due  consideration  of 
the  work  of  other  investigators,  has  endeavored  to  present  a  point  of 
view  that  has  been  prompted  by  years  of  personal  observation  and 
thought,  and  which  at  the  same  time  gives  a  simple  and  comprehensive 
explanation  of  the  largest  number  of  facts.  For  purposes  of  illustra- 
tion constitution  formulas  have  been  made  use  of,  such  as  the  author 
has  previously  employed  in  his  monograph,  "Zur  Erkenntnis  der 
Kolloide," 

The  hypothesis,  that  the  electric  charge  on  irreversible  hydrosols  is 
due  to  the  adsorption  of  ions  on,  or  the  giving  up  of  ions  by,  the  parti- 
cles, seems  capable  of  explaining  an  enormous  number  of  experimental 
facts.  The  principles  of  this  point  of  view,  arising  out  of  Hardy's 
work,  were  first  presented  by  Bredig,  and  more  fundamentally  dealt 
with  by  Billitzer.  The  author  had  independently  employed  the  same 
hypothesis  to  explain  peptisation  and  the  reactions  of  the  purple  of 
Cassius.  These  considerations  were  not  published  until  1904. 

Cotton  and  Mouton  favored  the  electric  double  layer  theory  of 
Helmholtz.  Duclaux,  who  with  Jordis  had  considered  hydrosols  of 

*  Arth.  Muller:  Die  Theorie  der  Kolloide.  Leipzig  u.  Wien  (1903).  Allge- 
meine  Chemie  der  Kolloide.  Leipzig  (1907).  Wo.  Ostwald:  Grundriss  der  Kolloid- 
chemie.  Dresden  (1909).  H.  Freundlich:  Kapillarchemie.  Leipzig  (1909).  W.  B. 
Hardy:  Zeit.  f.  phys.  Chemie,  33,  326-343,  385-400  (1900).  G.  Bredig:  Anor- 
ganische  Fermente.  Leipzig  (1901).  F.  G.  Doiman:  Zeit.  f.  phys.  Chemie,  37, 
735-743  (1901);  46,  197-212  (1903).  J.  Billitzer:  Zeit.  f.  phys.  Chemie,  46,  307- 
330  (1903);  61,  129-166  (1905).  E.  Jordis:  Neue  Gesichtspunkte  zur  Theorie  der 
Kolloide.  Berichte  d.  phys.-med.  Sozietat  Erlangen,  36,  43-107  (1904).  Koll.- 
Zeit.,  2,  1-7;  3,  1-27  (1908).  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide.  Jena 
(1905).  A.  Cotton  et  H.  Mouton:  Les  ultramicroscopes  et  les  objects  ultramicro- 
scopiques.  Paris  (1906).  L.  Michaelis:  Physikalische  Chemie  der  Kolloide. 
Leipzig  (1908).  P.  P.  von  Weimarn:  Zur  Lehre  von  den  Zustanden  der  Materie. 
I.  Teil.  Dresden  (1908).  II.  Teil  in  Koll.-Zeit.,  3  u.  6  (1908  u.  1909).  J.  Du- 
claux: Journ.  de  Chim.  Phys.,  7,  405-463  (1909).  G.  Malfitano:  Kolloidchem. 
Beihefte,  2,  142-212  (1910).  A.  Lottermoser:  Zeit.  f.  phys.  Chemie,  60,  451-463 
(1907);  62,  35^-383  (1908);  70,  239-248  (1910).  Koll.  Zeit.,  6,  78-83  (1910). 

71 


72  CHEMISTRY  OF  COLLOIDS 

the  type  of  iron  oxide  as  solutions  of  complex  ions,  later  accepted  the 
theory  that  electrolytically  charged  particles  were  present.  Duclaux  has 
dedicated  some  very  careful  work  to  the  theory  of  the  osmotic  pressure 
and  the  conductivity  of  irreversible  sols.  Further,  the  adsorption  of 
ions  has  been  assumed  by  Freundlich  and  Lottermoser,  while  the 
dissociation  of  molecules  on  the  surface  of  the  particles  has  been  advo- 
cated by  L.  Michaelis.  - 

Because  of  the  lack  of  space  the  theory  of  the  electric  double  layer 
will  not  be  taken  up  in  detail.  Neither  will  there  be  any  lengthy  dis- 
cussion of  the  question  raised  by  Freundlich  as  to  whether  the  adsorbed 
ions  are  .on  the  surface  of  the  particles,  or  in  the  water  layer  immedi- 
ately surrounding  the  particles.  From  the  results  obtained  by  Freund- 
lich the  last  supposition  seems  a  very  probable  one,  but  there  is  no 
necessity  for  discussing  the  problem  here. 

All  the  fundamental  methods  employed  in  colloidal  chemistry,  the 
Tyndall  effect,  ultramicroscopy,  ultrafiltration,  and  finally  the  appli- 
cation of  osmotic  pressure  point  unmistakably  to  the  conclusion  that 
the  particles,  although  they  are  at  times  very  small,  are  nevertheless 
much  larger  formations  than  the  molecules  of  crystalloids. 

The  first  question  that  presents  itself  is  the  cause  of  the  stability  of 
colloidal  solutions.  It  is  advisable  to  consider  the  two  great  classes 
separately;  the  reversible,  those  that  subdivide  spontaneously;  and 
the  irreversible,  those  that  lack  the  property  of  spontaneous  subdivision. 

1.  Reversible  Colloids.  —  The  causes  for  the  stability,  and  also  the 
solubility,  of  hydrosols  of  the  reversible  colloids  is  not  clearly  under- 
stood. The  solubility  probably  is  occasioned  by  the  same  influences 
that  govern  the  solubility  of  crystalloids.  Reversible  colloids  may 
possess  a  solution  tension  and  the  subdivision  would  therefore  occur 
because  of  diffusion.  Although  this  may  play  a  part,  it  is  not  by  any 
means  the  only  factor  at  the  root  of  the  spontaneous  subdivision,  be- 
cause many  reversible  colloids  subdividing  very  easily  in  a  medium, 
diffuse  with  difficulty  and  exhibit  a  smaller  osmotic  pressure  than  many 
irreversible.  The  behavior  of  PaaPs  colloidal  gold,  Lea's  silver  and 
Kollargol  may  be  cited  as  examples.  A  negative  surface  tension  be- 
tween the  colloid  and  the  water  may  also  be  assumed  and  the  dissolu- 
tion laid  to  this  cause.*  A  very  important  and  illuminating  piece  of 
experimental  work  pointing  toward  this  view  under  the  assumption  of 
Laplace's  theory  of  capillarity  has  been  performed  by  Donnan.f  More- 
over, the  adsorption  of  water  is  doubtless  at  the  bottom  of  the  disten- 

*  E.  Riecke:  Lehrbuch  der  Physik,  1,289  (3.  Aufl.).  van  der  Mensbrugghe:  Con- 
gr£s  international  de  Physique,  T.  I.,  487  (1900). 

t  F.  G.  Donnan:  Zeit.  f.  phys.  Chemie,  46,  197-212  (1903). 


THEORY  73 

tion  of  reversible  colloids.  Perhaps  a  chemical  combination  between 
the  ultramicrons  and  the  water  also  plays  an  essential  role.  Many  of 
these  colloids  have  the  property  of  causing  irreversible  colloids  to 
become  reversible. 

2.  Irreversible  Colloids.  —  These  bodies  do  not  have  the  property  of 
going  into  solution  spontaneously.  The  conditions  governing  the  sta- 
bility of  their  hydrosols  are  given  in  the  chapters  on  metal  hydrosols. 
The  most  interesting  point  for  consideration  here  is  the  charge  on  the 
particles,  which  is  fundamental  for  the  stability  of  the  hydrosol.  If 
the  particles  become  discharged  the  hydrosol  coagulates  and  the  precipi- 
tate is  always  isoelectric  with  the  medium.  This  has  been  doubted  by 
Billitzer  who  held  that  the  minimum  stability  and  the  isoelectric  point 
did  not  necessarily  fall  together.  Later  experiments  by  Burton,  Sved- 
berg,  and  von  Galecki  have  confirmed  the  earlier  results  of  Hardy. 
Neutral  particles  may  be  charged  positively  with  acid,  and  negatively 
with  alkalis.  On  this  fundamental  proposition  of  Hardy's  may  be 
based  the  most  useful  theories  of  coagulation  and  peptisation. 

The  causes  for  the  electric  charges  on  the  particles  have  already  been 
discussed  in  Chapter  III.  We  will  merely  call  attention,  here  to  the 
following  possibilities  which  are  of  first  importance  for  this  part  of 
the  discussion. 

The  Taking  up  and  the  Giving  off  of  Ions. —  It  is  the  concensus 
of  opinion  among  a  large  number  of  investigators  that  the  adsorption 
of  ions  plays  an  essential  role  in  the  charge  of  colloids.  Since  the 
publication  of  the  fundamental  work  of  Hardy,  the  author  has  been 
much  occupied  with  this  question,  and  has  come  to  the  conclusion  that 
the  adsorption  and  dissociation  of  ions  by  the  particles  offer  the  only 
rational  explanation  of  the  coagulation  and  peptisation  by  electrolytes. 

Formation  of  Chemical  Compounds.  —  Certain  experimenters  hold 
that  the  hydrosols  of  silicic  acid,  stannic  acid,  etc.,  do  not  contain  these 
substances  as  such,  but  are  chemical  combinations  in  solution.  Ac- 
cording to  this  point  of  view  colloidal  silicic  acid  is  either  a  solution  of 
alkali  silicate  having  a  high  molecular  weight  and  a  large  excess  of 
silicic  acid,  or  a  combination  of  hydrochloric  and  silicic  acid.  Simi- 
larly stannic  acid  is  a  stannate  of  high  molecular  weight.  Against  this 
viewpoint  many  objections  may  be  raised. 

1.  In  the  first  place  one  would  have  to  assume  that  chemical  com- 
pounds may  have  an  almost  continually  increasing  molecular  weight. 
This  would  entail,  according  to  amount  of  sodium  or  chloride  present, 
the  inclusion  in  the  category  of  chemical  compounds  of  a  large  number 
of  badly  defined  bodies,  and  load  chemistry  with  much  useless  ballast. 
It  would  be  necessary  to  assume  the  presence  of  compounds  having 


74  CHEMISTRY  OF  COLLOIDS 

unusually  high  molecular  weights;  for  instance  one  molecular  weight 
of  sodium  to  500  or  1000  molecular  weights  of  silicic  acid.  See  Chapter 
VII  on  the  hydrosols  of  silicic  acid. 

2.  There  would  be  almost  no  upper  limit  to  molecular  weights  if  the 
submicroscopical  particles  were  regarded  as  ions  of  salts. 

3.  The  hypothesis  is  not  sufficient  to  explain  the  alkali  solubility 
of  the  purple  of  Cassius,  and  similar  colloidal  complexes. 

Although  this  point  of  view  does  not  sufficiently  explain  the  reactions 
of  irreversible  colloids,  it  must  be  admitted  that  there  is  a  strong  re- 
semblance between  some  of  these  solutions  and  those  of  complex  salts. 
The  change  from  colloidal  oxides  to  true  electrolytes  is  so  gradual  that 
no  sharp  line  can  be  drawn  between  them.  It  is  quite  possible  that 
some  of  these  solutions  contain  electrolytes  of  high  molecular  weight, 
and  they  do  not  diffuse  through  membranes  because  of  the  size  of  the 
molecules  and  ions.  Such  could  be  true  in  the  case  of  reversible  oxides 
containing  a  great  deal  of  peptising  agent,  and  also  in  the  case  of  some 
colloidal  dye  stuffs. 

In  the  following  paragraphs  a  few  specific  examples  will  be  given  to 
show  how  peptisation  and  a  number  of  other  colloidal  reactions  may  be 
explained  in  a  simple  manner  on  the  basis  of  adsorption  of  ions. 

Theory  of  Peptisation 

The  theory  of  peptisation  must  be  preceded  by  a  brief  consideration 
of  the  history  of  the  ultramicrons  during  the  formation  of  a  gel.  Char- 
acteristic ultramicrons  can  be  observed  in  a  hydrosol,  in  the  precipi- 
tated hydrogel,  and  once  more  in  the  hydrosol  after  peptisation.  The 
logical  conclusion  to  draw  from  this  fact  is  that  the  particles  too  small 
to  be  seen  by  the  ultramicroscope  behave  similarly.  In  other  words 
the  particles  in  a  hydrogel  are  identical  with  those  in  the  hydrosol 
with  the  exception  that  they  are  much  closer  together  in  the  former. 
This  is  confirmed  in  the  case  of  the  gel  Of  silicic  acid  where  the  struc- 
ture is  much  finer  than  was  assumed  from  the  experiments  of  Butschli. 
See  Chapter  VII,  on  the  gel  of  silicic  acid;  also  the  chapter  on  the 
change  of  color  of  colloidal  gold;  peptisation  of  the  purple  of  Cassius. 
The  fact  that  hydrogels  are  so  easily  turned  into  hydrosols  is  additional 
evidence  that  the  particles  do  not  unite  to  form  larger  complexes  when 
the  gel  is  formed  from  the  hydrosol. 

We  must  picture  to  ourselves  a  gel  as  consisting  of  a  closely  compact 
mass  of  ultramicrons  of  the  same  kind  holding  each  other  together  by 
cohesion.  The  interstices  of  a  newly  prepared  gel  are,  of  course,  filled 
with  water.  It  is  only  necessary  to  add  an  electrolyte,  one  ion  of  which 
is  more  adsorbed  than  the  other,  in  order  to  recharge  the  particles,  and 


THEORY  75 

therefore  cause  peptisation.     Let  us  choose  stannic  acid  on  which  to 
further  illustrate  the  action. 

If  alkali  is  added  to  a  gel  of  stannic  acid  without  doubt  a  small 
portion  of  each  unite  to  form  a  stannate.  It  is  equally  certain  that  a 
part  of  the  stannate  will  be  adsorbed  by  the  stannic  acid.  If  now  the 
mixture  is  diluted  with  water  the  dissociation  of  the  stannate  occurs, 
and  one  has  but  to  assume  that  the  stannate  ion  is  more  adsorbed  by 
the  ultramicrons  than  the  potassium  ion  in  order  to  explain  the  pep- 
tisation. The  stannate  ion  imparts  the  negative  charge  to  the  parti- 
cles and  the  subdivision  goes  on  in  a  manner  very  similar  to  that  .of 
diffusion.  The  potassium  ion  diffuses  to  portions  of  the  liquid  where 
the  concentration  is  not  so  great  taking  in  its  wake  the  stannate  ion 
with  its  attendant  ultramicrons,  just  as  in  the  case  of  hydrochloric  acid 
the  hydrogen  ion  leads  the  way  to  places  of  lower  concentration  and 
the  chloride  ion  follows.*  The  reaction  may  be  expressed  by  means  of 
the  following  equation: 


SnQ2|  +  Sn03K2  =  |Sn02[  Sn03K2 


where  |  SnQ2  [  represents  an  ultramicron  regardless  of  the  real  size,  form, 
or  composition.     The  reaction  of  dilution  may  be  represented  thus: 


Sn02|  Sn03K2  <±  |Sn02|  Sn03=  +  2  K+  (B) 


The  complex  |SnQ2  Sn03—  has  the  properties  of  a  complex  ion  having 


two  or  more  valences,  and  differs  from  it  only  in  regard  to  size.     It  may 
or  may  not  be  hydrated. 

Under  certain  circumstances  a  very  small  amount  of  alkali  is  suffi- 
cient to  cause  complete  peptisation  without  any  further  dilution.  The 
direct  adsorption  of  the  stannate  ion  may  be  assumed.  In  this  case 
the  following  reactions  come  to  pass. 

1.  Diffusion  of  the  KOH  into  the  interior  of  the  clots  and  the  sub- 
sequent formation  of  K2Sn03,  the  anion  of  which  is  strongly  adsorbed. 
As  a  consequence  of  this  reaction  the  concentration  of  the  K+  is  in- 
creased in  the  interior  of  the  clots. 

2.  When  the  concentration  of  the  K+  is  sufficiently  great,  and  the 
particles  correspondingly  charged,  the  attraction  between  the  particles 
is  lessened  owing  to  increased  osmotic  pressure  in  the  interior  of  the 
clots.     See  page  82  on  the  theory  of  membrane  equilibria. 

3.  Diffusion  of  the  charged  ultramicrons  and  the  potassium  ion 
molecules  into  the  surrounding  liquid. 

Instead  of  the  formation  of  the  free  stannate  and  the  subsequent 
adsorption  of  the  anion,  one  can  just  as  well  assume  that  the  stannate 

*  W.  Nernst:  Theoretische  Chemie,  page  367,  5.  Aufl. 


76  CHEMISTRY  OF  COLLOIDS 

is  formed  on  the  surface  of  the  ultramicrons,  and  that  dissociation  takes 
place  here  leaving  the  anion  on  the  particles.  It  may  also  be  assumed 
that  the  ionization  on  the  surface  of  the  ultramicrons  takes  place  thus  : 


|  Sn03H2  +  20H-  +  2K+<=»|  Sn03=  +  2K+  +  2H20. 

^  represents  a  stannic  acid  molecule  on  the  surface  of  an  ultra- 
micron.  As  there  is  no  means  of  deciding  between  these  assumptions 
and  they  all  lead  to  the  same  result,  viz:  the  charging  of  the  ultrami- 
crons and  the  subdivision  of  the  material,  it  seems  easier  to  abide  by 
assumption  one;  that  is  the  adsorption  of  the  ions  by  the  colloidal 
particles.  The  fact  that  this  assumption  is  the  more  general,  and  that 
it  may  be  employed  in  explaining  the  formation  of  mixed  colloids,  is  a 
further  reason  for  making  it.  See  Chapter  VII  on  Peptoids. 

If  now  an  amount  of  alkali  insufficient  to  cause  peptisation  is  added 
most  of  it  is  taken  up  by  the  stannic  acid,  without  entailing  any  appar- 
ent outward  change.  On  adding  somewhat  more  alkali  a  cloudy 
liquid  is  formed,  the  ultramicrons  of  which  are  composed  of  a  number  of 
amicrons.  When  an  excess  of  the  alkali  is  now  added  the  amicrons 
may  suffer  a  further  subdivision,  and  finally  be  completely  transformed 
into  starinate.  This  process  has  nothing  whatever  to  do  with  true 
peptisation. 

The  subdivision  of  the  ultramicrons,  the  "Anatzen"  of  a  hydrogel, 
should  by  no  means  be  regarded  as  it  has  been  by  many  authors.  This 
follows  from  a  consideration  of  the  small  amount  of  alkali  necessary 
to  cause  peptisation.  As  shown  by  experiment  the  alkali  causes  a 
very  small  diminution  in  the  linear  dimensions  of  the  ultramicrons  pro- 
vided that  all  of  them  are  attacked  simultaneously.  According  to  the 
"Atztheorie"  it  is  not  at  all  clear  why  such  an  insignificant  sub- 
division should  cause  such  a  radical  change  in  the  properties.  This 
latter  theory  also  does  not  take  any  account  of  the  charges  on  the 
particles. 

If  the  theory  we  have  adopted  is  correct,  the  colloidal  solution  must 
exhibit  osmotic  pressure  against  a  membrane  impermeable  to  the  ami- 
crons of  the  stannic  oxide  complex;  it  must  possess  conductivity  and 
precipitate  SnC>2  at  the  anode  during  electrolysis,  and  toward  electro- 
lytes must  behave  similarly  to  a  complex  salt  of  high  molecular  weight. 
That  all  this  is  true  in  reality  will  be  shown  in  what  follows. 

Nature  of  the  Adsorbed  Ions.  —  The  electric  charge  is  equally  well 
explained  by  the  assumption  that  any  anion  other  than  the  stannate 
ion  is  adsorbed.  In  the  case  under  consideration  the  hydroxide  ion 
might  be  taken  up.  In  view  of  the  following  reactions  it  seems  more 
desirable  to  assume  the  adsorption  of  the  stannate  ion. 


THEORY  77 

1.  The  addition  of  an  excess  of  potassium  hydroxide  causes  the  pre- 
cipitation of  the  colloidal  stannic  acid.     The  ultramicrons  are  discharged 
and  potassium  ion  is  taken  up.     Therefore  reaction  (B),  page  75,  is 
written  reversibly. 

2.  The  discharged  ultramicrons  are  precipitated  with  the  adsorbed 
stannate.     A  similar  reaction  occurs  when  potassium  chloride  or  any 
other  potassium  salt  is  added. 

3.  If  the  hypothesis  is  correct  the  precipitated  stannic  acid  must  go 
back  into  solution  when  the  precipitating  agent  is  removed,  because  of 
the  renewed  dissociation  of  the  adsorbed  stannate.     This  is  indeed  the 
case. 

4.  An  extremely  small  amount  of  any  highly  dissociated  acid,  such 
as  HC1  or  HN03  serves  to  coagulate  the  hydrosol  of  stannic  acid. 


[Sn02]  $uOj=  +  2  H+  =  |SnO2|  SnO3H2. 


According  to  the  equation  just  given  stannic  acid  is  formed,  which  is 
very  insoluble  and  has  very  little  tendency  toward  dissociation.  The 
gel  should  therefore  remain  as  such  and  not  subdivide  on  the  removal  of 
the  precipitating  reagent.  This  has  been  found  to  be  the  case. 

5.  The  addition  of  the  salts  of  heavy  metals,  such  as  silver  nitrate, 
causes  irreversible  precipitation.  The  precipitate  contains  a  small 
amount  of  the  cation  that  cannot  be  washed  out.  The  precipitate  is 
practically-  insoluble  in  pure  water  because  the  stannate  is  itself  an 
insoluble  salt. 


Sn02   Sn08=  +  2  Ag+  -»  M  SnO2  SnO3Ag2. 


Doubtless  the  real  reactions  are  much  more  complicated  than  is  indi- 
cated by  the  equation,  nevertheless  it  serves  very  well  to  indicate  the 
trend  of  events  in  a  wide  field  that  is  not  very  easily  comprehended. 

Consideration  of  the  Behavior  of  Hydrosols 

A.  Ultrafltration.  —  By  the  use  of  filters  that  allow  electrolytes  to 
pass  freely  through,  but  retain  the  colloidal  particles,  colloidal  stannic 
acid  must  have,  after  filtration,  not  only  its  ultramicrons,  with  their 
attendant  anions,  but  also  an  equivalent  amount  of  alkali  ion  mole- 
cules. The  excess  of  the  electrolytes,  KOH,  K2SnOa,  etc.,  that  were 
dissolved  in  the  disperse  medium,  have  passed  through.  The  adsorbed 
portion  of  the  alkali,  regardless  of  whether  it  is  dissociated  or  not,  is 
an  essential  part  of  the  hydrosol;  for  if  it  is  removed  the  colloid  will 
coagulate.  Duclaux,  who  has  studied  the  behavior  of  colloidal  iron 
oxide  and  cupric  ferrocyanide  in  this  connection,  has  proposed  the  name 
"Micells"  for  the  ultramicrons  together  with  their  adsorbed  molecules 


78  CHEMISTRY  OF  COLLOIDS 

and  dissociation  products;  while  the  surrounding  medium  he  calls  the 
intermicellular  -liquid. 

B.  With  reference  to  the  conductivity  of  colloidal  solutions  and  the 
filtrates  from  such,  the  following  may  be  said: 

1.  If  the  ion  concentration  of  the  intermicellular  liquid  is  large  com- 
pared to  that  of  the  micells,  the  conductivity  of  the  filtrate  will  not 
differ  very  much  from  that  which  it  was  before  the  ultrafiltration. 
That  is,  the  conductivity  of  the  filtrate  and  the  residue  has  the  same 
value.     Such  cases  have  been  observed  by  Malfitano.    See  Chapter 
VII  on  colloidal  iron  oxide. 

2.  On  the  other  hand,  if  the  ion  concentration  of  the  intermicellular 
liquid  is  relatively  small,  the  conductivity  of  the  filtrate  will  be  lower 
than  that  of  the  residue.    Also  the  conductivity  of  the  residue  will  in- 
crease with  increasing  concentration,  because  both  the  ultramicrons 
and  the  attendant  oppositely  charged  ions  take  part  in  the  transporta- 
tion of  electricity.     Duclaux  observed  an  increase  in  conductivity  in 
the  residue  after  it  had  been  filtered  from  the  liquid. 

C.  Osmotic  Pressure.  —  The  residue  must  exert  osmotic   pressure 
against  the  filtrate.     According  to  the  explanation  on  page  75,  with  suit- 
able membranes  not  only  the  dissolved  molecules,  but  also  the  colloidal 
particles  (even  suspended  particles  to  a  certain  degree),  must  exhibit 
osmotic  pressure.     That  is  to  say  every  particle  reacts  as  a  molecule 
does,  the  difference  being  merely  one  of  degree. 

In  the  case  we  have  been  studying  not  only  must  the  complex 
|  SnO2 1  Sn03—  exert  osmotic  pressure,  but  also  the  potassium  ion  that  was 
adsorbed  during  the  process  of  the  neutralization  of  the  ultramicrons. 
The  total  pressure  would  therefore  correspond  to  the  number  of  parti- 
cles in  the  unit  volume  were  it  not  for  the  fact  that  the  intermicellular 
liquid  in  its  partition  between  the  filtrate  and  the  residue  causes  com- 
plications. See  Chapter  VII.  In  general  this  influence  is  small  when 
the  concentration  of  the  intermicellular  electrolyte  is  low  compared  to 
that  of  the  colloid. 

Duclaux  has  further  observed  that  the  osmotic  pressure  of  colloidal 
iron  oxide  after  ultrafiltration  increases  faster  than  is  proportional  to 
the  concentration.  He  explains  this  on  the  assumption  that  the  ions 
dissociated  from  the  particles  form  an  electric  double  layer  around  the 
ultramicrons  and  that  the  whole  functions  as  an  individual  particle. 
Not  until  the  concentration  becomes  sufficiently  great  for  the  spheres 
of  influence  of  the  individual  ultramicrons  to  cut  and  cross,  can  the  ions 
of  the  micells  take  part  in  osmotic  pressure. 

D.  The   Effect   of  Electrolysis.  —  During   electrolysis   the   complex 
ISnOJ  Sn03=  goes  to  the  anode,  is  there  discharged  and  precipitated. 


THEORY  79 

The  cathion  migrates  toward  the  cathode  where  hydrogen  is  discharged 
and  set  free.  An  interesting  reaction  comes  to  pass  when  a  piece  of 
parchment  membrane  is  inserted  between  the  electrodes  in  the  solution. 
The  ultramicrons  cannot  pass  through  but  are  discharged  on  the  mem- 
brane and  form  a  gel.  To  explain  this  it  may  be  assumed  either  that 
the  adsorbed  anion  is  set  free  at  the  membrane,  passes  through  and 
continues  on  toward  the  anode;  or  that  the  discharge  takes  place  be- 
cause ions  of  opposite  charge  meet  at  the  membrane.  A  closer  study 
of  this  phenomenon  is  desirable. 

E.  Equivalence.  —  The  equivalence  referred  to  in  Chapter  III  be- 
tween the  different  amounts  of  ions  necessary  to  cause  precipitation 
is  easily  explained.  The  adsorbed  cathions  serve  merely  to  neutralize 
the  charges  on  the  anion,  and  the  two  must  therefore  be  equivalent 
when  the  same  amount  of  the  hydrosol  is  taken.  For  the  sake  of  vivid- 
ness the  reaction  on  a  single  particle  may  be  represented  by  the  reac- 
tions: 

fSnOTl  SnO3=  - 


SnO, 


Sn02|  Sn03Ag2 


Sn02|  Sn03Cu 


The  quantities  of  Cu++  and  A^  necessary  to  neutralize  the  charge  on  a 
given  ultramicron  are  equivalent  to  each  other.  In  a  manner  quite 
as  simple  the  equivalence  between  the  ions,  that  may  be  substituted  for 
each  other  in  the  precipitate,  may  be  explained. 

Peptisation  of  Stannic  Acid  by  Hydrochloric  Acid 

Not  only  may  the  peptisation  of  stannic  acid  by  alkali  be  explained 
on  the  above  assumption,  but  also  the  peptisation  by  means  of  hydro- 
chloric acid  is  accounted  for  on  the  same  grounds.  It  is  well  known 
that  metastannic  acid  is  insoluble  in  concentrated  hydrochloric  acid, 
and  that  on  dilution  it  will  dissolve  to  form  a  cloudy  liquid,  which  has 
colloidal  properties.  The  reaction  is  often  met  with  in  analytical 
chemistry.  The  concentrated  acid  doubtless  forms  a  stannic  chloride 
and  an  oxychloride.  If  we  assume  that  the  latter  is  SnOCl2  and  is 
adsorbed  to  a  considerable  degree  by  the  metastannic  acid,  the  pepti- 
sation on  dilution  is  explained  by  the  fact  that  SnOCl2  dissociates, 
leaving  the  cathion,  probably  SnO"^,  on  the  colloidal  particles  of  stannic 
acid,  thus  causing  them  to  be  positively  charged.  The  reaction  may  be 
represented  thus: 

Sn02  +  2  HC1  <±  SnOCl2  +  H2O 

[SnO^I  +  SnOCl2^>  |SnO2]  SnOCl2 
SnO,  SnOCl2  <=±  SnQ    SnO++  +  2  Cl= 


80  CHEMISTRY  OF  COLLOIDS 

Peptisation  of  Other  Colloids 

The  peptisation  of  most  oxides,  sulfides  and  salts  may  be  explained 
on  the  same  basis.  According  to  Graham  the  peptisation  of  ferric 
hydroxide  hydrogel  takes  place  on  treating  it  with  ferric  chloride. 
One  may  assume  that  the  ferric  ion  attaches  itself  to  the  ultramicrons 
of  the  colloidal  ferric  hydroxide,  thus  imparting  to  the  latter  the  positive 
charge. 

iFe^a]  +  Fe+++  ->  |Fe^A|  Fe+++ 

Or  one  can  also  assume  that  an  oxychloride  is  formed  and  that  the 
cathion  is  adsorbed  by  the  ultramicrons  of  the  gel,  as  represented  by 
the  following  equation. 

|Fe203[  +  Fe202++  -»  |Fe2O3  Fe202++ 

All  the  essential  qualities  of  the  colloid  are  accounted  for  by  either  of 
these  assumptions. 

Arsenious  sulfide,  like  many  other  sulfides,  is  peptised  by  hydrogen 
sulfide.  It  is  probable  that  the  hydrogen  sulfide  ion,  SH~,  is  responsi- 
ble for  the  negative  charge. 

As2S3  +  SH  -->  |As2S3|  SH~. 

In  the  case  of  arsenious  sulfide  sulfoarsenious  acid  may  be  formed 
and  the  anion  adsorbed  by  the  particles.  At  present  it  is  difficult  to 
determine  which  of  these  points  of  view  is  the  correct  one.  In  any 
event  the  negative  charge  is  occasioned  by  adsorption  of  ions,  or  by 
ionization  on  the  surface  of  the  particles.  The  colloidal  chemistry  of 
the  future  will  undoubtedly  have  to  deal  with  the  adsorption  of  ions  by 
ultramicrons. 

Colloidal  Salts.  —  In  a  great  many  other  cases  also  the  use  of 
structural  formulas  serves  to  elucidate  the  processes  and  the  reactions. 
In  the  chapters  on  colloidal  salts  other  instances  will  be  given.  Here  a 
single  illustration  will  be  taken  up,  viz:  the  peptisation  of  cupric  ferro- 
cyanide  gel  by  potassium  ferrocyanide. 


FeCy6Cu2|  +  FeCy6  =  +  4  K+  -»  |FeCy6Cu2|  FeCy  ==+  4  K 


The  cupric  ferrocyanide  colloidal  particles  adsorb  the  ferrocyanide  ion, 
become  negatively  charged  as  a  result  and  go  into  solution. 

Peptisation  of  Colloidal  Combinations 

Not  only  can  the  peptisation  of  simple  colloidal  oxides,  sulfides,  etc., 
be  explained  by  the  above  theory,  but  also  that  of  colloidal  mixtures 
or  "Kolloidverbindungen."  The  purple  of  Cassius  has  been  chosen 
as  a  representative  of  this  class.  Here  the  purely  chemical  theory, 


THEORY  81 

that  regards  the  irreversible  hydrosols  as  solutions  of  salt-like  sub- 
stances having  an  amphoteric  character,  fails  to  satisfy.  The  purple 
of  Cassius  is  composed  of  colloidal  gold  and  colloidal  stannic  acid,  and 
these  two  components  remain  together  throughout  all  the  reactions  un- 
less the  purple  of  Cassius  is  actually  destroyed.  See  Chapter  V,  on  the 


purple  of  Cassius.     Let  the  formula,   |  Au  |  Sn07   represent  a  particle 


of  this  substance.  There  is,  of  course,  no  justification  for  assuming  that 
the  complex  particle  contains  only  one  of  each  of  the  components. 
Furthermore  nothing  whatsoever  is  postulated  with  regard  to  size, 
amount,  shape,  or  position  in  space.  The  addition  of  alkali  causes 
the  peptisation  as  a  result  of  the  negative  charge  in  precisely  the  same 
manner  as  in  the  case  of  stannic  acid. 


Au|SnO2|  +  K2Sn03-»  |Au|SnO2|  SnO3K2 


Au|Sn02[  Sn03K2  ^  |Au|Sn02|  Sn03= 


Although  peptised  purple  of  Cassius  behaves  in  solution  as  a  high 
molecular  complex,  it  cannot  be  considered  as  such  because  the  gold 
retains  its  elementary  nature.  See  Chapter  V. 

Transitional  Stages  Between  Electrolytic  Solutions  and  Irrever- 
sible Hydrosols 

There  are  so  many  transitional  stages  between  electrolytic  solutions 
and  irreversible  hydrosols  that  no  sharp  dividing  line  may  be  drawn. 
Let  us  consider  an  aqueous  solution  of  iron  chloride  as  it  is  diluted  suc- 
cessively. At  great  dilution  a  yellowish  brown  precipitate  of  ferric 
hydroxide,  or  the  gel  of  iron  oxide  is  obtained.  It  has  not  yet  been 
determined  whether  the  precipitate  is  Fe(OH)3  or  amicrons  of  Fe203 
separated  from  each  other  by  spaces  filled  with  water.  For  the  sake 
of  simplicity  we  will  assume  the  latter,  as  van  Bemmelen  has  done, 
without  committing  ourselves  to  either  hypothesis. 

Iron  oxide,  or  a  hydrated  oxide,  is  formed  from  ferric  chloride  even 
in  concentrated  solutions.  At  the  same  time  a  number  of  ferrioxy- 
chlorides  are  formed  as  a  result  of  hydrolysis  and  dissociation;  the 
constitution  and  composition  of  these  are  not  very  well  known.  The 
amicrons  of  iron  oxide  find  plenty  of  cathions  to  adsorb,  and  from  which 
they  can  get  the  positive  charge.  The  particles  do  not  unite  but  re- 
main in  solution.  As  the  dilution  becomes  greater  more  iron  oxide  is 
formed  and  either  unites  with  the  amicrons  to  increase  the  mass,  or 
goes  to  form  more  amicrons  which  in  turn  adsorb  more  cathions.  When 
the  dilution  has  become  sufficiently  great  the  majority  of  the  cathions 
of  the  ferric  chloride,  or  oxychloride  are  adsorbed.  Further  dilution  or 


82  CHEMISTRY  OF  COLLOIDS 

the  addition  of  alkali  results  in  the  discharge  of  the  colloidal  particles 
because  of  the  adsorption  of  hydroxyl  ions.  A  similar  process  comes  to 
pass  when  a  solution  of  ferric  chloride  or  nitrate  is  subjected  to  dialysis. 
At  higher  temperatures  the  hydrolysis  is  much  more  pronounced. 
Even  when  the  concentration  is  fairly  great  a  hydrosol  may  be  obtained. 
Witness  the  appearance  of  the  brown  color  on  warming  a  solution  of 
ferric  nitrate.  Long  boiling  effects  a  complete  hydrolysis  of  ferric 
acetate.  Here  the  conditions  are  favorable  for  the  formation  of  large 
particles,  and  the  amicrons  grow  into  the  submicroscopic  field.  The 
hydrosols  are  strongly  turbid  owing  to  the  presence  of  meta-iron  oxide 
and  the  submicrons  exhibit  a  crystalline  character,  as  demonstrated  by 
Cotton  and  Mouton. 

Theory  of  Membrane  Equilibria  in  the  Presence  of  Electrolytes 
that  Cannot  be  Dialyzed  Out 

Donnan  *  has  investigated  the  equilibria  that  result  when  an  ionic 
combination  is  shut  off  from  migrating  electrolytes  by  a  semipermeable 
membrane. 

A.  Division  of  an  Electrolyte  with  a  Common  Ion.  —  Assume  the  com- 
plete dissociation  of  the  salt  NaR  into  the  ions  Na+  and  R~.  The  solu- 
tion is  to  be  separated  from  sodium  chloride  solution  by  a  membrane 
impermeable  to  the  ion  R~.  The  system  may  be  schematically  repre- 
sented thus: 


(1) 

Na+ 

R- 


(2) 

Na+ 

cr 


Sodium  chloride  would  diffuse  from  (2)  to  (1)  and  we  should  then  have 
the  equilibrium: 


(1) 
Na+ 
R- 

cr 


(2) 

Na+ 


cr 


The  work  necessary  for  the  isothermal,  reversible  transportation  of  one 
mol  of  sodium  ion  from  (2)  to  (1)  is  exactly  equivalent  to  that  work 
gained  by  the  transportation  of  one  mol  of  Cl  under  the  same  condi- 
tions. The  total  work  gained,  or  the  loss  of  free  energy,  is  therefore 
zero. 


[Na+]2  X  [Cr|s  =  [Na+t  X  [CHi,  (1) 

*  F.  G.  Donnan:  Zeit.  f.  Elektrochemie,  17,  572-581  (1911). 


THEORY 


83 


wherein  the  brackets  represent  molar  concentrations.     Complete  dis- 
sociation and  equal  volumes  on  either  side  the  membrane  are  assumed. 

Original  state  Equilibrium 

Na+  Cl" 

C2  C2 


|    Na+    BT  Cl 

|   Ci  -f-  X      Ci        X 


Na+      Cl~ 

C2  —  X     Cz  —  X 


Na+  R~ 

Ci         Ci 

(1)  (2)  (1)  (2) 

Ci  and  c2  are  the  molar  concentrations  of  the  respective  ions. 

100  r 

^-Z  is  the  per  cent  of  NaCl  that  diffuses  from  (2)  to  (1). 

is  the  relation  of  NaCl  on  the  two  sides  of  the  membrane  at 

x 

equilibrium. 
From  equation  (1)  we  have  the  relation 

(ci  +  x)  x  =  (c2  —  x)2    or    x 


d  +  2  c2 


TABLE  12 


Original  concen- 
tration of  NaR  in 
1 

Original  concen- 
tration of  NaCl 
in           2 

Original  relation  of 
NaR  to  NaCl, 

Per  cent  NaCl 
diffused  from  1  to  2, 
100  x 

Final  distribution 
relation  of  NaCl 
between  2  and  1, 

Cl 

C2 

C2 

C2 

x 

0.01 

1 

0.01 

49.7 

1.01 

0.1 

1 

0.1 

47.6 

1.1 

1 

1 

1 

33 

2 

1 

0.1 

10 

8.3 

11 

1 

0.01 

100 

1 

99 

From  the  table  it  will  be  seen  that  in  case  the  concentration  of  NaR  is 
large  in  comparison  to  that  of  NaCl  the  latter  will  scarcely  penetrate 
the  membrane.  On  the  contrary,  if  the  concentration  of  NaCl  is  very 
large  compared  to  that  of  NaR,  the  latter  will  scarcely  affect  the  diffu- 
sion of  the  sodium  chloride. 

B.  Osmotic  Pressure.  —  The  unequal  division  of  NaCl  influences  the 
measurement  of  the  osmotic  pressure.  Let  Po  be  the  true  osmotic 
pressure  of  NaR,  and  PI  the  observed  osmotic  pressure.  According  to 
Donnan  we  obtain  the  following  equation  : 

Pi         Ci  +  C2 


In  case  Ci  is  small  compared  to  Cz,  PI  =  |  P0.  On  the  other  hand  if  eg 
is  small  compared  to  Ci,  PI  =  PO.  In  other  words  the  osmotic  pressure 
of  the  salt  that  does  not  penetrate  the  membrane  is  unaffected  if  its 
concentration  is  great  compared  to  that  of  the  diffusing  salt. 


84 


CHEMISTRY  OF  COLLOIDS 


C.  Division  of  an  Electrolyte  without  a  Common  Ion.  —  If  KC1  is 
substituted  for  NaCl  in  the  considerations  under  A  we  come  to  the 
following  result. 

If  the  concentration  of  NaR  is  large  compared  to  KC1,  most  of  the  K+ 
will  diffuse  through  to  (1);  only  a  small  amount  of  Cl~  will  go  from  (2) 
to  (1),  and  a  small  part  of  the  sodium  will  go  from  (1)  to  (2).  On  the 
contrary,  if  the  contraction  of  KC1  is  large  compared  to  that  of  NaR, 
an  appreciable  quantity  of  Na  will  go  from  (1)  to  (2). 

D.  Hydrolytic  Decomposition  of  Salts  by  a  Membrane.  —  If  pure  water 
is  put  on  one  side,  (2),  of  the  membrane  and  the  solution  of  NaR  on 
the  other,  (1),  the  sodium  ion  has  a  tendency  to  go  through  to  (2). 
This  is  possible  only  when  the  hydroxide  ion  also  diffuses  from  (1)  to 
(2).     The  solution  in  (1)  becomes  acid  while  that  in  (2)  must  be  alkaline. 
The  relations  may  be  represented  thus  : 


Original  State 

(D  (2) 


Na+ 


Pure  water 


Equilibrium 

(1)  (2) 

Na+     H+  R-  Na+  OH' 

Ji  —  X     X        Ci         X  X 


(2) 


From  considerations  similar  to  those  made  in  deriving  equation  (1) 
we  obtain: 

[Na+]i      [PET], 
[Na+]2      [OH-]! 

Assuming  complete  dissociation  of  the  electrolytes  and  the  same  vol- 
ume on  each  side  of  the  membrane,  the  following  relations  obtain  for 
equilibrium : 

d  —  x          x 


Therefore 


x          [OH]! 
x  X  [OH-]  =  K* 


If  x  is  small  compared  to  Ci,  then 


x  = 


Under  these  conditions  the  hydrolysis  of  NaR  is  very  small  as  evidenced 
by  the  following  table. 


TABLE  13 


100  x 

Ci 

I 

Cl 

0.01 

5.10-6 

Per  cent 

0.05 

0.1 

1.10-* 

0.01 

1 

2  10-* 

0.002 

THEORY  85 

By  increasing  the  volume  of  (2)  the  hydrolysis  may  be  increased. 
When        vz  =  100  vl  and  Ci  =  0.1, 


If  the  dissociation  constant  of  HR  is  very  small,  or  the  acid  very 
insoluble  the  hydrolysis  will  be  greater.  Donnan  has  made  calculations 
for  several  such  cases. 

E.  lonization  of  an  Acid,  the  Anion  of  which  Cannot  Pass  Through  a 
Membrane.  —  When  the  pure  acid  is  in  the  compartment  (1)  and  so- 
dium hydroxide  in  (2)  much  of  the  latter  will  dialyze  from  (2)  to  (1). 
A  non-dialyzable  salt  will  be  formed  in  (1).  If  the  acid  was  originally 
in  the  form  of  a  gel  or  tiny  crystals,  it  would  exhibit  no  osmotic  pressure 
against  pure  water.  On  the  other  hand,  if  sodium  hydroxide  is  added 
to  (2)  osmotic  pressure  will  be  manifested  at  once.  Donnan  has  cal- 
culated the  potential  differences  at  equilibrium  for  several  such  cases. 
These  calculations  are  important  in  considerations  concerning  the 
nerves,  and  for  the  explanation  of  the  electrical  organs  in  many  fishes. 

Donnan's  theory  is  also  applicable  to  complex  ions  having  a  high 
molecular  weight,  where  there  is  little  tendency  to  penetrate  mem- 
branes. 


CHAPTER  V 
INORGANIC   COLLOIDS* 

A.   Colloidal  Metals 

General  Properties.  —  Those  colloidal  metals  that  are  free  from 
protective  colloids  will  be  treated  first.  They  occur  often  in  sub- 
microscopical  form,  have  the  most  varied  colors,  and  are  particularly 
suitable  for  ultramicroscopic  investigation.  In  spite  of  this  property 
they  may  be  obtained  in  a  nearly  homogeneous  form  if  sufficient  care 
is  exercised  during  the  preparation.  Under  certain  circumstances 
scarcely  a  ray  of  light  is  discernible  even  under  the  ultramicroscope. 
They  cannot  be  prepared  in  concentrated  form,  generally  under  0.1 
per  cent.  If  the  endeavor  is  made  to  concentrate  them  either  by 
evaporation,  ultrafiltration,  or  by  treatment  with  electrolytes,  coagu- 
lation invariably  results.  They  are  almost  always  negatively  charged 
in  neutral  or  very  weakly  alkaline  or  acid  solution.  Under  suitable 
conditions,  sometimes,  they  can  be  made  to  separate  out  at  the  cathode. 
Generally,  however,  any  attempt  to  change  their  direction  results 
merely  in  the  complete  discharge  of  the  particles  and  the  subsequent 
precipitation  of  the  colloid.  In  these  cases  the  precipitation  laws  of 
Schulze  and  Hardy  hold  approximately. 

The  most  characteristic  property  of  this  class  is  the  strong  tendency 
among  the  particles  to  unite  with  one  another. f  In  fact  the  preparation 
of  colloidal  metals  may  be  regarded  as  an  interrupted  condensation 
process.  This  tendency  is  so  pronounced  that  even  energetic  dis- 
solution may  occasion  coagulation.  Opposed  to  this  is  the  property 
of  forming  colloidal  combinations  with  protective  colloids.  The  com- 
plex has  many  of.  the  properties  and  exhibits  the  reactions  of  pro- 
tective colloids.  Sufficient  weight  has  not  always  been  given  to  this 
behavior  in  the  study  of  reversible  colloids.  The  fact  that  metal 
colloids  may  imitate  the  reactions  and  properties  of  the  protective 
colloid  when  only  traces  of  the  latter  are  present,  without  losing  their 
own  optical  properties,  demonstrates  quite  clearly  the  fallacy  of  the 
hypothesis  which  says  that  the  particles  of  protective  colloids  are  fine 
and  all  alike  in  size.  It  is  equally  untenable  to  consider  them  as  molec- 

*  Chapters  V  to  IX  inclusive. 

f  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  141  (1905). 
86 


INORGANIC  COLLOIDS  87 

ular  divisions  of  the  nature  of  crystalloids.  For  instance,  colloidal 
gold  to  which  gelatin  to  the  amount  of  3  per  cent  of  the  weight  of 
gold  has  been  added,  resembles  gelatin  in  its  properties.  It  can  be 
precipitated  by  tannic  acid  but  not  by  table  salt.  Practical  use  is 
made  of  the  above  mentioned  property  in  the  determination  of  the 
protective  action  of  reversible  and  irreversible  colloids  toward  gold. 
The  determination  of  the  gold  number  helps  to  classify  the  protective 
colloids.  See  gold  number,  page  106. 

In  spite  of  the  fact  that  some  authors  have  questioned  whether  col- 
loidal metals  really  belong  to  colloidal  solutions,  contending  that  they 
are  in  reality  suspensions,  nevertheless  this  class  has  contributed  greatly 
toward  our  knowledge  of  the  entire  field.  In  addition  to  what  has 
already  been  said,  Bredig's*  article  on  the  analogy  between  colloidal 
metals  and  ferments  may  be  cited.  Colloidal  metals  have  been  the 
occasion  of  many  other  advances  and  have  stimulated  the  develop- 
ment of  ultramicroscopy.  The  confirmation  of  the  kinetic  theory 
resulted  from  Svedberg'sf  work  on  the  Brownian  movement.  More- 
over Biltz  {  has  shown  that  the  mutual  precipitation  of  colloidal  metals 
with  positively  charged  hydrosols  cannot  be  explained  on  the  assump- 
tion of  a  salt  formation;  and  in  general  that  during  the  precipitation 
of  colloids,  as  well  as  in  the  case  of  dyestuffs,  chemical  and  also  specific 
reactions  are  involved.  The  synthesis  of  the  purple  of  Cassius  from 
colloidal  gold  has  been  of  importance  for  the  theory  of  colloidal  chem- 
istry. The  peptisation  of  this  purple  has  rendered  the  purely  chemical 
point  of  view  of  peptisation  untenable.  As  shown  by  Luppo-Cramer  § 
many  of  the  phenomena  of  photography  can  be  explained  on  the  basis 
of  considerations  concerning  the  behavior  of  colloidal  metals.  The 
photochlorides  of  Carey  Lea  have  been  shown  by  R.  Lorenz  and  Luppo- 
Cramer  If  not  to  be  subchlorides  but  adsorption  complexes. 

Preparation.  —  Pure  metal  hydrosols  are  prepared  either  by  reduc- 
tion of  the  corresponding  salt  in  dilute  solution,  by  electric  colloidation 
according  to  Bredig  and  The.  Svedberg,  or  by  light  reactions.  For 
instance,  Svedberg  ||  prepared  silver  hydrosol  by  the  action  of  ultra- 
violet light  on  a  silver  plate  submerged  in  water.  The  process  is  ap- 
plicable to  lead,  copper,  and  tin,  but  fails  with  aluminium  or  gold. 

*  G.  Bredig:  Anorganische  Fermente.     Leipzig  (1901). 

t  The.  Svedberg:  Studien  zur  Lehre  von  den  kolloiden  Losungen,  125-160. 
Upsala  (1907). 

t  W.  Biltz:  Ber.,  37,  1095-1116  (1904). 

§  Luppo-Cramer:  Kolloidchemie  und  Photographic.     Dresden  (1908). 

1"  M.  Carey  Lea  und  Luppo-Cramer:  Kolloides  Silber  und  die  Photokloide. 
Dresden  (1908). 

[I  The.  Svedberg:  Ber.,  42,  4375-4377  (1909);  Koll.-Zeit.,  6,  129-136  (1910). 


88  CHEMISTRY  OF   COLLOIDS 

Siedentopf  succeeded  in  making  silver  bromide  hydrosol  by  means  of 
light  through  the  cardioid  ultramicroscope.* 

Protected  Colloids.  —  Combinations  of  metal  colloids  and  protec- 
tive colloids  have  often  been  classed  with  metal  colloids.  They  will 
be  dealt  with  later,  but  are  of  sufficient  interest  to  warrant  a  few  re- 
marks here.  To  this  class  belongs  Lea's  colloidal  silver,  and  Paal's 
colloidal  metals  f  that  show  the  interesting  catalytic  effect  in  reduc- 
tion reaction  with  hydrogen;  for  instance,  the  formation  of  succinic 
acid  from  fumaric  acid  and  the  preparation  of  stearic  acid  from  oleic 
acid. 

The  practical  applications  of  colloidal  metals  are  many;  an  instance 
in  point  is  KuzeFs  lamps.  Protected  metal  colloids  have  been  used  in 
medicine  but  there  seems  to  be  a  difference  of  opinion  as  to  their 
efficacy. 

1.  PURE  METAL  COLLOIDS 

Conditions  for  Stability.  —  Attention  must  be  again  called  to  the 
sensitiveness  toward  electrolytes  of  pure  metal  colloids,  very  small 
quantities  of  electrolytes  sufficing  to  cause  precipitation.  The  question 
naturally  arises  why  these  colloids  are  so  easily  coagulated  when  those 
of  the  albumin  type  are  so  stable  toward  electrolytes.  Generally  this 
question  is  set  aside  with  the  reply  that  colloidal  metals  more  nearly 
resemble  suspensions  than  do  the  hydrofiles.  The  answer,  however, 
does  not  satisfy,  for  a  small  quantity  of  protective  colloid  renders 
colloidal  metals  just  as  stable  toward  electrolytes  as  albumin  is.  The 
degree  of  dispersion  is  not  changed  by  the  addition  of  the  protective 
colloid;  the  particles  must  increase  their  size  and  mass  somewhat  and 
therefore  more  nearly  approach  the  suspensions.  Furthermore  amicro- 
scopic  gold  hydrosols  are  almost  as  sensitive  to  electrolytes  as  those 
whose  particles  are  much  larger.  Hence  any  explanation  of  electrolyte 
sensitiveness  on  the  basis  of  the  degree  of  dispersion  is  unsatisfactory. 
Equally  untenable  is  the  assumption  made  by  Donnan,{  to  explain  the 
dissolution  of  colloids,  that  there  is  an  attraction  between  the  particles 
and  the  surrounding  medium.  A  feasible  theory  offers  an  opportunity 
to  the  theoretical  physicist  and  has  not  yet  been  presented.  At  present 
we  must  content  ourselves  with  the  hypothesis  in  accordance  with  the 
facts,  that  the  attraction  of  the  particles  for  one  another  is  great  enough 
to  cause  an  irreversible  union  if  the  ultramicrons  come  sufficiently 
close  together,  as  they  must  do  during  ultrafiltration  or  centrifugali- 

*  Page  14. 

t  C.  Paal  und  C.  Amberger:   Ber.,  37,  124-139  (1904):   38,  1398-1405  (1905); 
40,  1392-1404  (1907). 
.    $  F.  G.  Donnan:  Zeit.  f.  phys.  Chemie,  37,  735-743  (1901);  46,  197-212  (1903). 


INORGANIC  COLLOIDS  89 

zation.  All  influences,  therefore,  that  render  it  more  difficult  for  the 
particles  to  come  within  the  critical  distance  (the  distance  at  which 
they  will  unite)  tend  to  make  the  system  more  stable.  Enumerated 
these  are: 

1.  The  average  distance  of  the  particles  from  one  another. 

2.  The  electric  charge  on  the  particles. 

3.  The  viscosity  of  the  medium. 

4.  The  union  of  the  metal  ultramicrons  with  protective  colloids. 

With  regard  to  the  effect  of  1  on  the  stability  we  know  from  experi- 
ment that  the  stability  of  gold  hydrosol  is  increased  on  dilution.  This 
is  probably  due  to  the  fact  that  the  particles  are  less  likely  to  come 
within  the  sphere  of  other  particles  when  the  distance  between  them 
is  great.  During  ultrafiltration,  evaporation,  and  centrifugalization  the 
particles  are  brought  close  together  and  coagulation  results.  In  other 
words  the  distance  between  the  particles  must  not  be  too  small  or  pre- 
cipitation takes  place  in  spite  of  the  electric  charge. 

The  influence  of  discharge  of  the  particles  has  been  dealt  with  in 
Chapter  IV.  In  all  cases  where  the  particles  are  discharged  coagula- 
tion results.  The  discharge  is  not  confined  to  the  influence  of  electro- 
lytes, but  may  be  brought  about  by  a  rays,  ultraviolet  rays,  etc.  In 
all  these  cases  the  precipitation  takes  place  without  a  necessary  change 
in  the  distance  between  the  ultramicrons. 

The  most  efficient  means  of  preventing  an  irreversible  union  be- 
tween the  particles  is  the  addition  of  protective  colloids.  The  true 
significance  of  this  will  be  considered  later.  It  might  be  remarked 
here  that  there  are  all  possible  grades  of  protective  effect.  Colloids 
which  give  scarcely  any  protection  against  electrolytes  may  prevent 
the  change  of  color  when  the  hydrosol  is  concentrated  in  collodion 
sacks.  It  is  possible  to  concentrate  a  colloidal  gold  solution  to  a  con- 
siderable degree  by  ultrafiltration  without  a  change  of  color,  where, 
under  similar  circumstances,  the  unprotected  colloid  would  change  its 
color  and  coagulate. 

Colloidal  Gold 
PREPARATION  AND  PROPERTIES 

Colloidal  solutions  of  metallic  gold  have  been  known  for  over  two 
hundred  years.  Ruby  glass  has  been  recognized  since  the  time  of 
Kunkel,  1679,  and  the  gold  purple  of  Cassius  since  1685.  In  both  prep- 
arations the  gold  is  in  the  colloidal  form,  although  this  fact  was  not  a 
matter  of  common  knowledge  until  recent  years.  Pure  gold  hydrosols 
were  first  made  by  Faraday  *  in  1857.  Although  he  did  not  realize 

*  Faraday:   Phil.  Transact.,  154  (1857). 


90  CHEMISTRY  OF  COLLOIDS 

the  relation  of  his  preparations  to  ruby  glass  and  the  purple  of  Cassius, 
the  work  is  so  fundamental  and  important  that  a  short  account  will  be 
given  here.  The  term  colloidal  solution  was  not  proposed  by  Graham 
until  a  year  later. 

Faraday  treated  dilute  gold  chloride  solutions  with  phosphorus  dis- 
solved in  ether  or  carbon  disulfide.  From  this  he  obtained  more  or 
less  turbid  solutions,  sometimes  purple  red,  sometimes  violet  or  blue. 
The  gold  settled  out  very  easily;  occasionally,  however,  the  solutions 
were  stable  for  several  months.  Faraday  set  himself  to  prove  that  the 
blue  or  red  constituent  of  these  solutions  was  metallic  gold  in  a  finely 
divided  state.  He  succeeded  in  showing  that  the  electrical  colloidation 
of  gold  in  air  or  hydrogen  gave  a  precipitation  on  glass  or  quartz  which 
had  the  same  red  or  blue  color  as  the  gold  solutions.  He  also  proved 
that  the  precipitate  from  his  solutions  behaved  exactly  like  gold.  From 
this  he  concluded  that  the  precipitate  was  not  a  chemical  combination 
of  gold.  He  did  not  carry  out  a  quantitative  analysis.  He  further 
allowed  the  condensed  rays  of  sunlight  to  pass  through  the  solutions 
and  noted  the  diffusion  of  light.  Sometimes  the  color  was  unmistak- 
ably that  of  metallic  gold.  He  also  convinced  himself  that  solutions 
such  as  potassium  chromate  did  not  give  the  same  diffusion.  Faraday 
did  not  conclude,  as  many  commenting  on  his  article  have  since  done, 
that  his  solutions  were  coarse  suspensions  of  gold,  probably  because  he 
continually  asked  himself  the  question  whether  he  had  succeeded  in 
preparing  gold  in  the  molecular  form.  He  considered  it  astonishing 
that  the  volume  of  the  precipitate  was  several  hundred  or  even  thou- 
sands of  times  that  which  it  would  be  in  the  more  compact  metallic 
form. 

The  work  of  Faraday  remained  unnoticed  for  over  forty  years. 
Even  those  who  were  studying  ruby  glass  and  the  purple  of  Cassius 
did  not  take  it  into  cognizance.  The  author  himself  was  not  familiar 
with  these  investigations  of  Faraday  when  the  former  began  his  work 
on  the  nature  of  the  purple  of  Cassius. 

The  author  *  has  worked  out  a  method  for  the  preparation  of  stable 
solutions  of  pure  colloidal  gold.  The  procedure  consists  in  reducing 
dilute,  slightly  alkaline  gold  solutions  at  boiling  temperature  with 
formaldehyde.  The  best  results  are  obtained  by  the  following  method: 
120  cc.  of  specially  distilled  water  are  brought  to  boiling  in  a  Jena 
glass  beaker.  During  the  heating  2.5  cc.  of  a  solution  of  HAuCU  •  4  H2O 
(6  gms.  to  the  liter)  and  3  to  3.5  cc.  of  a  0.18  normal  solution  of  potas- 
sium carbonate  are  added.  As  soon  as  the  boiling  point  has  been 

*  R.  Zsigmondy:  Liebigs  Annalen,  301,  30  (1898);  Zeit.  f.  analyt.  Chemie,  40, 
711  (1901). 


INORGANIC  COLLOIDS  91 

reached  3  to  5  cc.  of  a  dilute  solution  of  formaldehyde  (0.3  cc.  of  ordi- 
nary concentrated  formalin  to  100  cc.  water)  are  added  with  violent 
shaking  or  stirring.  Soft  glass  stirring  rods  are  to  be  avoided.  In  a 
few  seconds,  or  at  longest  one  minute,  an  intense  red  color  appears  and 
does  not  change  on  further  standing.  The  water  used  in  this  prepara- 
tion is  procured  by  redistilling  water  in  a  silver  condenser;  a  Jena 
glass  receptacle  is  employed  to  catch  the  water  from  the  silver  con- 
denser. All  the  liquids  necessary  for  preparing  the  gold  solutions  will 
keep  indefinitely  so  that  one  to  two  liters  of  the  desired  solutions  may 
be  prepared  in  one  hour's  time.  They  are  also  very  cheap,  containing 
about  4  cents  worth  of  gold  to  the  liter. 

In  this  manner  deep  red  or  purple  red  colloidal  gold  solutions  of  great 
stability  are  obtained.  The  particles  are  generally  visible  under  the 
ultramicroscope,  the  size  being  from  10  to  40  ^.  The  different  solu- 
tions prepared  by  this  method  will  not  be  exactly  alike  unless  great 
care  is  exercised  in  the  preparation  of  the  water.  These  solutions  may 
be  used  to  determine  the  gold  number  of  other  colloids. 

If  solutions  containing  finer  subdivisions  are  desired  a  much  greater 
dilution  is  necessary  when  formaldehyde  is  employed  as  the  reducing 
agent.  The  dilution  should  be  increased  to  TTT  o5uTny  per  cent,  but  fortu- 
nately the  concentration  may  be  made  100  times  greater  by  subsequent 
evaporation.  The  particles  in  such  solutions  are  amicroscopic.  A 
much  simpler  preparation  *  has  been  developed  which  is  a  combination 
of  the  above  with  the  method  of  Faraday.  A  gold  chloride  and  potas- 
sium carbonate  solution  is  prepared  by  the  above  recipe  and  a  few 
drops  of  a  solution  of  phosphorus  in  ether  are  added.  The  reducing 
agent  is  best  prepared  by  diluting  a  saturated  phosphorus  solution  in 
ether  to  about  five  times  its  volume.  About  \  cc.  of  this  will  be  sufficient 
for  the  purpose.  If  the  mixture  is  allowed  to  stand  for  several  hours 
it  turns  blue,  brown,  or  sometimes  black.  In  24  hours  it  will  have 
gradually  become  red,  and  contains  particles  too  small  to  be  detected 
by  the  ultramicroscope.  Often  not  a  trace  of  the  light  rays  may  be 
seen.  The  red  color  appears  more  quickly  if  the  mixture  is  boiled. 
The  boiling  may  be  continued  until  all  the  ether  is  driven  off,  and  air 
may  be  bubbled  through  to  oxidize  the  phosphorus  without  causing 
any  change  in  the  hydrosol  itself. 

A  third  method, t  which  makes  it  possible  to  procure  solutions  of 
particles  of  almost  any  desired  size,  consists  in  a  combination  of  the 
two  already  given.  A  colloidal  solution  of  the  finest  subdivision  is 
prepared  by  the  second  method.  These  solutions  may  be  called  nuclear 

*  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  100  (1905). 
t  R.  Zsigmondy:  Zeit.  f.  phys.  Chemie,  56,  65-76  (1906). 


92  CHEMISTRY  OF  COLLOIDS 

solutions  or  nuclear  liquids.  A  second  mixture  is  prepared  from  potas- 
sium carbonate  and  gold  chloride  in  exactly  the  same  proportions  as  in 
the  first  method.  Formaldehyde  is  now  added  but  before  the  reduction 
begins  a  definite  quantity  of  the  nuclear  liquid  is  also  added  to  the 
hot  mixture.  It  is  advantageous  to  add  water  so  that  the  reduction 
without  the  nuclear  liquid  would  occupy  about  two  minutes.  The 
reduction  without  the  nuclear  liquid  would  take  place  slowly  and  the 
result  would  be  a  turbid  purple  colored  solution.  When  the  nuclear 
solution  is  added  the  reduction  is  almost  instantaneous  and  a  clear 
deep  red  gold  solution  is  obtained.  The  size  of  the  particles  may  be 
regulated  somewhat  by  the  amount  of  nuclear  liquid  added,  a  small 
amount  aiding  in  the  formation  of  larger  particles,  while  a  large  amount 
may  leave  the  ultramicrons  still  amicroscopic.*  It  is  obvious  that  the 
liquids  must  be  free  from  any  substances  that  would  act  in  a  manner 
detrimental  to  the  nuclear  solution. 

The  explanation  of  the  effect  of  the  nuclear  liquid  is  doubtless  that 
the  particles  act  as  centers  around  which  the  condensation  takes  place, 
just  as  occurs  in  the  formation  of  crystals.  The  individual  particles 
grow  in  the  mixture  as  long  as  there  remains  any  gold  to  be  reduced. 
If  there  are  a  great  many  nuclei  present  the  supply  of  gold  will  be  ex- 
hausted quickly  and  the  particles  cannot  become  so  large  as  they  would 
if  few  nuclei  were  added. 

Not  only  may  the  size  of  particles  in  gold  solutions  be  thus  regulated 
in  order  to  obtain  solutions  for  specific  purposes,  but  this  same  method 
may  also  be  employed  to  determine  the  dimensions  of  the  amicrons  in 
the  original  nuclear  liquid. 

The  most  concentrated  solutions  made  by  one  or  other  of  these 
methods  contain  from  5  to  7.5  mgs.  of  gold  per  100  cc.  The  volume 
may  be  decreased  to  about  one-half,  but  further  decrease  of  volume 
usually  results  in  coagulation  because  of  the  increasing  concentration 
of  the  dissolved  electrolytes.  If  more  concentrated  gold  solutions  are 
desired  dialysis  must  be  resorted  to  in  order  to  lessen  the  amount  of 
electrolytes  present.  For  this  purpose  the  dialyzer  should  be  kept  in  a 
warm  place  where  there  is  a  good  circulation  of  air  to  aid  in  the  evap- 
oration. In  this  manner  it  is  possible  to  increase  the  concentration  from 
10  to  20  times.  The  author  has  succeeded  in  preparing  solutions  con- 
taining 0.12  per  cent  of  gold  by  this  method.  If  the  concentration  is 
carried  far  enough  shining  gold  rings  may  be  obtained  on  the  spherical 
shaped  parchment  membrane.  The  gold  thus  formed  will  amalgamate 
slowly  with  the  vapors  of  mercury. 

Colloidal  solutions  are  tasteless  and  non-poisonous.     The  gold  is 

*  W.  Menz:  Zeit.  f.  phys.  Chemie,  66,  132  (1909). 


INORGANIC  COLLOIDS  93 

precipitated  by  a  series  of  bases,  acids,  and  salts.  Contrary  to  the 
effect  of  most  electrolytes  potassium  cyanide  does  not  produce  coagu- 
lation in  .dilute  gold  solutions  for  the  reason  that  it  partially  dissolves 
the  gold,  and  consequently  makes  the  color  lighter.  With  more  con- 
centrated gold  solutions  potassium  cyanide  causes  the  color  to  become 
blue,  and  precipitation  follows.  Poorly  dissociated  electrolytes,  such  as 
acetic  acid  or  ammonium  hydroxide,  have  a  small  precipitating  effect. 

Other  methods  for  the  preparation  of  gold  solutions  by  chemical 
means  have  been  proposed  from  time  to  time  without  bringing  out  any- 
thing very  new.  Gutbier  *  employed  hydrazine,  Donau  f  carbon 
monoxide,  and  Doerinckel  {  hydrogen  peroxide  as  reducing  agents. 
These  methods  have  the  advantage  that  they  prevent  the  possible 
formation  of  protective  colloids.  One  might  also  reduce  gold  solutions 
with  sugar,  phenols,  aromatic  aldehydes,  ethereal  oils,  etc.;  but  for 
many  purposes  it  is  inadvisable  to  introduce  foreign  substances  into 
the  gold  hydrosol.  Blake  §  used  an  ether  solution  of  acetylene  to  re- 
duce the  gold  and  obtained  fairly  concentrated  solutions  which  he 
employed  for  experiments  on  coagulation. 

An  electrolytical  method  for  the  preparation  of  colloidal  gold  origi- 
nated with  Bredig.lf  An  electric  arc,  about  1  mm.  long,  between  gold 
points  is  formed  under  water.  Four  to  five  amperes  at  110  volts  may 
be  used,  but  8  to  10  amperes  at  70  volts  are  more  satisfactory.  The 
gold  solution  thus  obtained  is  blue  or  violet  blue  in  color  and  is  free 
from  electrolytes.  Purple  red  solutions  are  made  from  these  prepa- 
rations by  adding  a  small  amount  of  alkali.  Hydrosols  prepared  by 
this  method  are  preferable  for  investigations  where  the  presence  of 
electrolytes  would  interfere  with  the  measurements,  such  as  the  de- 
termination of  the  conductivity,  and  also  where  dissolved  substances 
may  act  as  catalytic  poisons. 

Paal's  method  for  preparing  gold  solutions  involves  the  use  of  pro- 
tective colloids,  and  will  be  discussed  in  the  chapter  on  platinum  col- 
loids. 

Evidence  of  the  Metallic  Nature  of  Colloidal  Gold 

While  Faraday  endeavored  to  establish  by  qualitative  experimen- 
tation that  his  gold  subdivision  was  metallic  gold,  the  direct  analysis 
was  first  made  on  the  precipitate  obtained  from  the  formaldehyde 

*  A.  Gutbier:  Zeit.  f.  anorg.  Chemie,  31,  448-450  (1902). 
t  J.  Donau:   Monatsh.  f.  Chemie,  26,  525-530  (1905). 
J  F.  Doerinckel:  Zeit.  f.  anorg.  Chemie,  63,  344-348  (1909). 
§  J.  C.  Blake:  Amer.  Journ.  of  Sc.  (4),  16,  381-387  (1903). 
H  G.  Bredig:  Zeit.  f.  Elektrochemie,  4,  514  (1898);  Zeit.  f.  angew.  Chemie,  951- 
954  (1898).     Anorganische  Fermente,  24.     Leipzig  (1901). 


94  CHEMISTRY  OF   COLLOIDS 

preparation  that  had  been  thrown  down  by  sodium  chloride.*  The 
precipitate  was  gathered  in  an  asbestos  filtering  tube,  dried  and  heated 
to  redness  in  a  current  of  carbon  dioxide.  The  resulting  gas  was  caught 
over  potassium  hydroxide  to  remove  the  carbon  dioxide,  and  the  re- 
mainder treated  with  pyrogallic  acid  and  phosphorus.  These  two  latter 
took  up  only  one-tenth  as  much  as  they  would  have  if  the  precipitate 
had  consisted  of  the  lowest  oxide  of  gold,  Au20.  Some  nitrogen  re- 
mained, showing  that  the  gold  had  adsorbed  a  small  amount  of  air.  Fur- 
ther, aurous  oxide,  Au20  reacts  with  HC1  according  to  the  following 
equation. 

3  Au20  +_6  HC1  =  4  Au  +  2  AuCl3  +  3  H2O. 

When  the  precipitate  was  treated  with  hydrochloric  acid  mere  traces 
of  gold  went  into  solution,  from  which  it  may  be  concluded  that  an 
extremely  small  amount  of  aurous  oxide  could  have  been  present.  Be- 
cause for  other  reasons  higher  oxides  of  gold  could  not  have  been  present, 
it  follows  that  the  precipitate  must  have  been  metallic  gold. 

J.  C.  Blake  f  prepared  colloidal  solutions  of  gold  by  reduction  with 
acetylene  dissolved  in  ether,  and  analyzed  the  precipitate  obtained  by 
treatment  of  the  hydrosol  with  barium  salts.  He  found  it  quite  pure 
or  contaminated  with  a  very  small  amount  of  barium  if  the  precipitate 
was  thrown  down  in  alkaline  solution.  There  were  also  traces  of 
carbon  which  doubtless  came  from  the  ether. 

A  few  words  may  be  added  with  regard  to  purple  gold  oxide,  the  ex- 
istence of  which  had  already  been  doubted  by  Proust,  Buisson,  Figuier, 
and  definitely  negated  by  Kreuss.J  The  assumption  was  made  to  ex- 
plain the  color  of  ruby  glass  and  the  purple  of  Cassius  at  a  time  when 
colloidal  solutions  of  gold  were  not  known. 

Theory  of  the  Color  of  Colloidal  Gold 

The  color  of  colloidal  gold  solutions  in  transmitted  light  may  be 
red,  violet,  or  blue,  and  occasionally  yellowish  brown,  or  brown.  The 
ultramicrons  of  red  solutions  are  green;  those  of  blue  solutions  are 
yellow  to  reddish  brown;  violet  solutions  contain  both.  We  have 
therefore  to  do  with  green,  yellow,  or  brown  ultramicrons.  There  is 
not  the  multiplicity  of  colors  here  that  there  is  in  the  case  of  silver 
solutions. 

Relations  Between  Color  and  Size  of  Particles.  —  Both  green  and 
brown  ultramicrons  may  have  all  possible  dimensions  from  the  amicro- 

*  R.  Zsigmondy:  Liebigs  Annalen,  301,  43  (1898). 

t  J.  C.  Blake:  Contributions  from  the  Kent  Chem.  Lab.  of  Yale  University, 
CXX.,  4  Ser,  16  (1903) 

J  G.  Kruess:  Liebigs  Annalen,  237,  274-307  (1887). 


INORGANIC  COLLOIDS  95 

scopic  to  120  w  and  over.  As  a  general  thing,  however,  the  large 
particles  are  yellow  or  brown  while  the  very  fine  subdivisions  are  green. 
At  present  there  is  no  explanation  for  the  fact  that  very  small  particles 
are  sometimes  brown.  Nevertheless  the  following  may  be  the  key  to 
the  situation.  According  to  Mie's  theory  particles  of  gold  having  a 
diameter  of  40  w  and  under  must  be  green.  The  assumption  is  thereby 
made  that  the  shape  is  spherical  and  the  particle  a  compact  mass  of 
metallic  gold.  Any  divergence  from  the  theory  may  mean  that  the 
conditions  are  not  fulfilled.  In  other  words  when  the  very  small  par- 
ticles are  brown,  either  the  shape  is  not  spherical  or  the  entire  space 
occupied  by  ultramicrons  is  not  filled  with  metallic  gold.  The  first 
assumption  does  not  seem  to  be  entirely  necessary.  It  may  also  be 
contended  that  the  divergence  from  the  theory  is  due  to  allotropic 
modifications  of  gold.  The  assumption  is  quite  unnecessary  for  the  ex- 
planation of  the  color  and  in  certain  cases  leads  to  contradictions. 

With  regard  to  the  brown  color  of  very  small  particles  a  large  number 
of  experimental  facts  point  to  the  assumption  that  the  ultramicrons 
are  not  composed  of  massive  gold.  For  instance,  whenever  the  green 
particles  become  flocculent,  or  approach  very  close  to  one  another,  the 
color  changes  to  brown,  even  when  the  aggregate  is  still  amicroscopic. 
It  would  seem  therefore  that  small  brown  particles  are  in  reality  con- 
glomerates of  the  green.  Green  ultramicrons,  on  the  other  hand,  are 
composed  of  compact  gold,  and  are  the  result  of  the  normal  growth  of 
amicroscopic  particles,  or  better,  perhaps,  they  are  tiny  crystals. 

The  assumption  that  the  particles  are  spherical  in  form  is  made 
solely  for  the  purposes  of  calculation,  and  a  number  of  facts  would  seem 
to  cliscredit  the  hypothesis.  The  very  great  independence  of  the 
color  on  the  diameter  makes  it  seem  plausible  that  ultramicrons  in  red 
hydrosols  are  not  necessarily  spherical  when  the  size  is  40  M/Z  and  under. 

Absorption  Spectra.  —  The  absorption  spectra  of  red  gold  solutions 
were  found  to  correspond  well  with  that  of  ruby  glass  first  by  the  author 
and  since  by  many  other  investigators.  In  deep  red  or  purple  red 
solutions  the  maximum  absorption  lies  near  the  spectral  line  E.  In 
the  case  of  blue  gold  solutions  the  maximum  lies  nearer  the  red  end  of 
the  spectrum.  The  absorption  band  is  also  wider,  and  the  refracted" 
light  from  the  particles  is  yellow  or  brown. 

Brown  Gold  Hydrosols.  —  Beside  blue  violet  and  red  gold  sub- 
divisions there  exist  others  that  appear  yellow  or  brown  in  transmitted 
light.  These  have  long  been  known  to  chemists.  Very  often  during 
the  preparation  of  the  purple  of  Cassius  from  SnCl2  and  HAuCL*  brown 
liquids  are  obtained  that  gradually  become  red.  Similar  results  are 
frequently  met  with  in  the  reduction  of  dilute  gold  chloride  solutions 


96  CHEMISTRY  OF  COLLOIDS 

by  phosphorus.  Gold  ruby  glass  containing  a  considerable  quantity 
of  lead  or  tin  often  solidifies  to  a  dark  yellow  or  brown  mass.  The 
author  has  prepared  yellowish  red  liquids  by  the  reduction  of  very 
dilute  solution  of  gold  chloride  with  formaldehyde.  Recently  Sved- 
berg  *  fundamentally  investigated  these  liquids.  Whether  the  color 
is  due  to  the  particles  of  gold  in  the  water  or  glass,  or  whether  it  is  a 
result  of  conglomerates  of  gold  and  PbO,  Sn02,  P,  etc.,  cannot  at  present 
be  decided  upon.  That  the  brown  color  usually  appears  when  foreign 
bodies  are  present  such  as  an  excess  of  phosphorus  in  the  reduction  of 
gold  chloride,  speaks  in  favor  of  the  assumption  that  conglomerates 
cause  the  brown  color. 

Theory  of  the  Colorations.  —  A  great  many  thorough  investiga- 
tions exist  on  the  theory  of  the  colorations  in  colloidal  metals.|  Ehren- 
haft  explains  the  color  on  the  assumption  of  an  optical  resonance  of 
the  particles.  Maxwell  Garnett  has  calculated  the  absorption  spectra 
of  gold  hydrosols  and  ruby  glass  by  means  of  Lorenz's  theory  {  for 
optical  inhomogeneous  media,  and  has  found  that  the  results  agree 
very  well  in  many  respects  with  those  obtained  by  direct  measurement. 
The  agreement  is,  however,  only  partial.  For  instance,  Maxwell 
Garnett  calculated  the  absorption  curve  for  gold  solutions  having 
molecular  dimensions  (dissolved  crystalloid)  and  found  a  maximum  at 

A  =  0.475  M. 

According  to  this  even  a  crystalloid  solution  of  metallic  gold  must  be 
colored.  Rapidly  cooled  ruby  glass  which  contains  metallic  gold  is, 
however,  colorless.  The  objection  to  this  that  the  gold  is  in  chemical 
combination  with  the  glass  is  untenable,  for  the  glass  remains  colorless 
on  rapid  cooling  regardless  of  the  presence  of  a  large  excess  of  reducing 
agent,  which  latter  must  have  freed  all  the  gold  from  combination. 

A  theory  that  explains  a  large  number  of  experimental  facts  has  been 
proposed  by  Mie.  Where  the  growth  of  the  particles  has  been  normal 
in  the  gold  solutions  the  polarization,  absorption  spectrum,  and  the 
diffusion  of  light  all  agree  well  with  the  theory.  Mie  gives  a  complete 
presentation  of  the  integration  of  Maxwell's  formula  for  a  sphere  and 
the  induction  of  a  sphere  by  a  direct,  linear  polarized  light  wave.  The 
absorption  in  the  gold  solution  is  calculated  from  the  loss  of  energy 
that  the  wave  experiences  in  the  sphere,  and  that  is  made  up  of  the 

*  The.  Svedberg:  Zeit.  f.  phys.  Chemie,  65,  624-633  (1909);  66,  752-758  (1909); 
67,  249-256  (1909);  74,  513-536  (1910). 

t  F.  Ehrenhaft:  Sitzungsber.  d.  Akad.  d.  Wiss.  Wien,  112,  lla,  181-209  (1903); 
114,  lla,  1115-1141  (1935).  F.  Kirchner  und  R.  Zsigmondy:  Drudes  Annalen  d. 
Phys.  (4),  15,  573-595  (1904). 

J  L.  Lorenz:  Wiedemanns  Annalen,  N.  F.,  11,  70-103  (1880). 


INORGANIC   COLLOIDS  97 

loss  through  heat  and  diffusion  of  light.  The  absorption  coefficient  is 
calculated  by  multiplying  the  energy  loss  on  one  particle  by  the  number 
of  particles.* 

The  absorption  coefficient  for  dilute  solutions  with  very  small    par- 
ticles is  according  to  Mie.f 


ntf  -h  n\ 

N  is  the  number  of  particles  in  a  cubic  centimeter;  V  is  the  volume  of 
a  particle;  X'  is  the  wave  length  in  water;  nQ  is  the  refractive  index  of 
water;  and  n\  is  the  complex  refractive  index  of  the  gold.  The  symbol 
Im  signifies  that  the  imaginary  part  of  the  complex  expression  is  to  be 
taken.  For  larger  subdivisions  the  refraction  coefficients  a,  6,  c,  d  must 
be  added.  The  above  is  an  approximation  formula,  for  a,  b,  c,  d  be- 
come equal  to  unity  when  the  particles  decrease  sufficiently  in  size. 
It  has  been  shown  that  the  absorption  curve  is  the  same  for  all  solutions 
of  gold  where  the  concentration  remains  constant  and  the  particles  are 
not  very  large.  This  agrees  very  well  with  the  facts,  for  gold  solutions 
differ  little  in  color  where  the  particles  lie  between  1  and  40  ju;u,  pro- 
vided that  the  growth  has  been  normal,  and  the  particles  have  not 
been  formed  by  the  union  of  two  or  more  others  as  in  coagulation. 

According  to  Mie,  Rayleigh's  formula  obtains  for  the  diffusion  of 
light  by  gold  solutions  having  very  small  particles. 

no2  -n/2 


X'4 

where  the  symbols  have  the  same  significance  as  before  and  the  vertical 
lines  indicate  that  the  absolute  value  of  the  expression  must  be  taken. 
For  larger  particles  a,  6,  c,  d  must  be  introduced.  The  value  of  the 
term  inside  the  vertical  lines  outweighs  that  of  the  well-known  factor 
in  Rayleigh's  formula,  so  that  the  curve  for  gold  solutions  differs  from 
that  of  isolated  particles  calculated  from  Rayleigh's  formula. 

From  the  formula  in  the  above  paragraph  it  may  be  concluded  that 
the  curves  for  different  sized  particles  are  similar,  and  that  the  ordi- 
nates,  where  the  gold  concentration  is  the  same  (N  V  is  a  constant), 
are  proportional  to  the  volumes  of  the  particles.  This  agrees  fairly 
well  with  the  experimental  facts. 

From  Fig.  16  which  was  constructed  from  measurements  by  Steu- 
bing,J  it  will  be  seen  that  the  diffusion  of  light  is  only  a  fraction  of  the 
absorption,  and  that  the  former  becomes  smaller  with  decrease  in  the 

*  F.  Hasenoehrl:  Sitzungsber.  d.  Akad.  d.  Wiss.  Wien,  111,  lla,  1229-1263  (1902). 

t  G.  Mie:  Koll.  Zeit.,  2,  129-133  (1907). 

J  W.'Steubing:   Drudes  Annalen  d.  Phys.  (4),  26,  329-371  (1908). 


98 


CHEMISTRY  OF  COLLOIDS 


size  of  the  particles.  While  the  absorption  of  liquids  having  ultra- 
microns  with  a  diameter  of  36  or  20  up  is  almost  the  same,  there  is  a 
marked  difference  in  the  intensity  of  the  diffusion.  The  latter  is  small 
for  particles  of  20  w  and  is  immeasurable  at  2  to  4  w 

Optically  those  gold  solutions  with  small  amicrons  behave  exactly 
as  homogeneous  solutions  of  dyestuffs.  A  part  of  the  absorption  is  to 
be  attributed  to  diffusion  of  light  only  when  the  subdivision  of  the 
gold  is  much  coarser.  The  diffusion  in  the  larger  subdivisions  has  an 
appreciable  influence  on  the  color.  In  the  case  of  the  finer  subdi- 
visions of  gold  hydrosols  the  color  is  not  a  question  of  the  color  of  a 


0.08 


0.07 


Absolution 


Diameter  otparttdeas  Pljij* 
do.  36/x.ft 

do.          ca.  20/ifi 
X  Absorption  calculated 
for  a  diameter  of  36/A/i. 


0.06 
0.05 
0.04 
0.03 
0.02 
0.01 


400/A/4  450  500  550  600  650  TOO,*/* 

FIG.  16.    Absorption  and  radiation  per  cc.  of  a  0.0025%  red  gold  solution. 

turbid  medium,  such  as  the  blue  of  the  heavens,  as  has  been  wrongly 
assumed  by  many,  but  is  rather  a  specific  absorption  of  ether  waves 
that  may  be  calculated  from  the  optical  constants  of  the  metal. 

Polarization  by  the  Particles.  —  Light  from  the  side  is  polarized  by 
the  ultramicrons.  The  smaller  the  particles  the  more  complete  is  the 
polarization,  and  this  is  linear,  not  elliptical.  Diagrams  17  and  18 
are  taken  from  an  elaborate  article  by  Mie  *  on  the  relation  of  the  size 
of  the  particles  to  the  polarization.  The  figures  hold  for  direct  rays  of 
sunlight.  The  intensity  of  the  diffracted  light  is  represented  by  the 
length  of  the  radial  vectors  from  the  particles.  The  outer  curve  cuts 
off  portions  of  the  radial  vectors  that  are  proportional  to  the  total 

*  G.  Mie:  Drudes  Annalen  d.  Phys.  (4),  25,  429  (1908). 


INORGANIC  COLLOIDS  99 

radiation.  The  inner  curve  gives  the  same  thing  for  the  nonpolarized 
light.  The  portion  between  the  two  curves  is,  therefore,  proportional 
to  the  polarized  light.  The  arrows  show  the  direction  in  which  the 
ray  is  traveling. 

From  the  figures  it  will  be  seen  that  the  very  small  gold  particles 
diffract  light,  that  the  latter  is  completely  linear  polarized  in  a 
direction  at  right  angles  to  the  path  of  the  ray,  and  that  in  all  other 
directions  the  light  is  only  partially  polarized. 


7          *      V 
FIG.  17.    Radiation  diagram  of  an  "  infinitely"  small  gold  particle. 

On  the  other  hand,  particles  of  160  MM  and  over  give  a  maximum 
polarization  at  120  degrees,  and  send  much  more  light  in  the  direction 
in  which  the  ray  is  traveling  than  in  any  other. 

The  theoretical  optics  of  metal  colloids  anticipates  many  phenomena 
that  are  found  in  practice.  A  special  instance  is  the  color  and  the 
polarization  of  particles  produced  by  normal  growth.  On  the  contrary, 

100 


120° 

FIG.  18.    Radiation  diagram  of  a  gold  particle  having  a  diameter  of  160  MM- 

a  complete  explanation  of  many  other  phenomena  is  wanting;  especially 
the  change  in  the  color  during  coagulation  to  a  yellowish  or  reddish 
brown,  regardless  of  whether  the  particles  are  amicroscopical  or  sub- 
microscopical.  There  are  many  facts  that  go  to  show  that  the  ultra- 
microns  are  not  isodimensional  in  their  contour,  but  are  long  and  flat 
or  leaf  shaped.  For  instance  the  dichroism  of  gold  gelatin  films  * 
*  The.  Svedberg:  Arkiv  for  Kemi,  4,  No.  19  (1911). 


100 


CHEMISTRY  OF  COLLOIDS 


must  be  attributed  to  a  difference  in  the  distance  between  the  particles 
in  the  two  directions,  or  to  a  definite  orientation  of  tiny  rods  or  leaves 
differing  lengthwise  of  the  film  from  that  which  they  have  at  right 
angles  to  the  film.  The  last  is  the  more  probable  because  all  gold 
hydrosols  are  not  suitable  for  showing  dichroism  with  gelatin  and  the 
property  is  manifested  in  a  marked  degree  only  by  those  containing 
anisotropic  particles.  Experiments  by  the  author  with  gold  solutions 
of  definite  amicrons  failed  to  show  any  dichroism.  It  might  also  be 
noted  that  gold  often  crystallizes  in  a  leaf-like  form  having  six  sides 
and  that  Ambronn  *  has  observed  dichromatic  microscopic  rods. 

A  strong  argument  that  the  color  of  the  ultramicrons  is  determined 
by  their  form  is  offered  by  the  experiments  of  Siedentopf.f  When 
gold  or  silver  ultramicrons  are  pressed  between  the  cover  glass  and  the 
platform  of  the  cardioid  ultramicroscope  the  green  or  variegated  particles 
become  brown.  The  color  change  may  be  caused  by  the  pressure  on 
small  cubes  as  well  as  by  the  orientation  of  tiny  flakes  or  rods.  Similar 
results  were  obtained  by  pressure  on  sodium  chloride  particles  colored 
with  sodium.  The  change  of  color  here  is  doubtless  due  to  the  pressure 
on  the  submicrons,  causing  them  to  assume  another  form. 

If  we  assume  a  flattening  effect  by  pressure  on  the  particles,  or  an 
orientation  of  the  flakes  or  rods  such  that  the  largest  surface  is  at 
right  angles  to  the  pressure,  we  may  represent  the  results  in  the  follow- 
ing scheme, t  Table  14. 

If  the  light  vibrations  from  the  polarizer  are  parallel  to  the  shorter 
diameter  of  the  particles,  that  is,  at  right  angles  to  the  flat  side,  then 
transmitted  light  is  red  and  diffracted  green. 

If,  on  the  contrary,  the  vibrations  are  parallel  to  the  larger  surface 
and  at  right  angles  to  the  shorter,  then  the  transmitted  light  is  blue 
and  the  diffracted  yellow  or  brown. 

TABLE  14 


Direction 
of  pressure. 

Supposed  posi- 
tion of 
particles. 

Plane  of 
vibration. 

Direction  of 
observation. 

Color  of  NaCl 
in  transmitted 
light. 

Color  of  NaCl  in 
reflected  light. 

— 

0 
0 

1 

• 

Red 
Blue 

Green 
Orange  brown 

1 
1 

0 
0 

1 

• 

Blue 
Red 

Orange  brown 
Green 

• 

0 
0 

1 

• 

Blue 
Blue 

Orange  brown 
Orange  brown 

*  H.  Ambronn:  Zeit.  f.  wiss.  Mikroskopie,  22,  349-355  (1905). 
t  H.  Siedentopf:  Verb.  d.  Deutsch.  Phys.  Ges.,  12,  6-47  (1910). 
Ibid. 


INORGANIC  COLLOIDS'  -'    V  ?        V  '  1Q1 


In  complete  accord  with  these  ideas,1 'are7  the  bbsef vations  «'of  Am- 
bronn  and  Zsigmondy  *  on  the  pleochroism  of  silver  or  gold  gelatin 
films,  for  which  the  first  named  offered  the  explanation.  Anisotropic 
metal  particles  (flakes  or  rods)  are  similarly  oriented  by  the  spreading 
out  of  the  gelatin.  If  the  vibrations  of  the  transmitted  light  are  parallel 
to  the  distention  direction  of  the  gelatin  the  color  is  blue.  If  the 
vibrations  are  at  right  angles  to  the  distention,  the  color  will  be  red. 
The  orientation  of  the  submicroscopic  rods  or  flakes  of  gold  is  such 
that  the  longest  diameter  is  parallel  to  the  direction  of  the  strain. 
This  is  to  be  expected  if  one  supposes  the  particles  to  occupy  small 
spaces  that  will  be  stretched  by  the  distention.  From  these  con- 
siderations it  follows,  as  Siedentopf  has  pointed  out,  that  the  theory  of 
Mie  must  be  expanded  to  include  rotation  ellipsoids.f 

Change  of  Color  During  Coagulation.  —  What  has  been  said  in  the 
foregoing  paragraphs  does  not  suffice  to  explain  all  the  phenomena 
encountered.  A  characteristic  property  of  all  pure  red  gold  solutions 
is  the  change  to  blue  during  coagulation.  This  change  is  occasioned  by 
the  union  of  two  or  more  particles  that  diffract  green.  The  complex 
thus  formed  diffracts  only  brown  light  waves.  It  is  impossible  to  ex- 
plain the  color  change  on  "the  grounds  of  an  increase  in  the  size  because 
it  occurs  regardless  of  whether  amicrons  or  submicrons  unite.  In  the 
first  of  these  cases  the  complex  may  still  remain  amicroscopic  and  have 
a  mass  several  hundred  times  smaller  than  that  of  a  large  red  particle. 
Nevertheless  these  tiny  complexes  diffract  brown  and  the  liquid  ap- 
pears blue. 

It  seems  necessary  to  assume  that  the  particles  unite  to  form  a  some- 
what loose  flocculent  mass,  and  do  not  melt  into  one  another  as  drops 
of  liquid  do.  For  if  the  latter  were  the  case  the  color  would  be  the 
same  for  all  particles  of  the  same  substance  having  like  dimensions. 
However,  as  already  stated,  there  is  no  relation  between  size  and  color 
unless  the  growth  has  been  normal;  that  is,  not  caused  by  union  of 
several  ultramicrons  larger  than  molecules. 

There  is  another  important  conclusion  to  be  drawn  from  the  con- 
siderations discussed  in  the  foregoing,  viz.,  that  a  decrease  in  the  surface 
is  not  a  very  prominent  factor  in  the  coagulation.  Even  if  one  assumes 
that  the  liquid  films  between  the  particles  are  broken  the  decrease  of 
surface  must  be  confined  to  the  edges  or  faces  that  touch. 

A  reversible  change  of  color  may  be  brought  about  by  evaporating  a 

*  H.  Ambronn:  Ber.  d.  Kgl.  Sachs.  Ges.  d.  Wiss.  Leipzig,  48,  Math.  phys.  KL, 
613-628  (1896). 

t  H.  Siedentopf:  Verb.  d.  Deutsch.  Phys.  Ges.,  12,  32  if.  (1910).  O.  Wiener: 
Phys.  Zeit.,  5,  332-338  (1904). 


102  CHEMISTRY  OF  COLLOIDS 

i         •>    o         »        0-1         •»  ,.  !*  -•>  S     •>  S  >         ^       '~fi 

*"*     -I  "'"l"""'*  i*  "S     *    ""*  «        "     «^  n 

colloidal  gold  solution  with  a  very  small  amount  of  gelatin.*  The  dried 
residue  is  blue  while  the  color  changes  to  red  if  moisture  is  added. 
The  change  of  color  has  been  explained  by  Kirchner  f  on  the  basis  of 
Plank's  t  dispersion  theory  for  iso tropic  dielectrics.  He  regards  the 
particles  as  resonators  that,  on  coming  into  close  proximity  with  one 
another,  displace  the  absorption  maximum  toward  the  red  end  of  the 
spectrum,  at  the  same  time  causing  a  widening  and  increased  intensity 
of  the  maximum.  This  is  borne  out  very  well  in  practice.  Mie  §  has 
raised  objections  to  Kirchner 's  theory  so  that  a  satisfactory  elucidation 
is  not  yet  at  hand.  A  complete  optical  theory  of  metal  colloids  must 
unquestionably  explain  the  change  of  color  that  is  so  characteristic 
of  gold  and  other  metal  colloids. 

Siedentopf  If  has  observed  an  unmistakable  dichroism  of  gold  gelatin 
films  when  viewed  at  an  oblique  angle.  He  assumes  that  the  change 
of  color  on  drying  is  due  to  a  change  of  form  of  the  particles  and  not 
to  the  distance  between  them.  It  is  difficult  to  conceive  of  a  reversible 
change  of  form,  however,  and  it  seems  much  better  to  assume  an  orien- 
tation of  the  particles  parallel  to  the  distention  surface  of  the  film. 
But  this  cannot  be  the  only  factor  involved  in  the  change  of  color  on 
dry  desiccation,  because  the  color  of  the  residue  seen  through  a  NichoPs 
prism  suitably  placed  is  a  turbid  violet  red  and  differs  greatly  from  the 
deep  red  obtained  by  the  addition  of  moisture.  The  distance  between 
the  particles  must  play  a  part  here  just  as  it  does  in  the  coagulation 
of  gold  solutions. 

A  word  may  be  added  with  regard  to  blue  gold  hydrosols.  The  blue 
obtained  on  the  reduction  of  gold  chloride  solutions  may  be  attributed 
to  three  causes.  First,  the  reduction  may  be  incomplete  and  colloidal 
gold  oxide  be  formed  instead  of  gold.||  Further  reduction,  perhaps  at 
higher  temperatures,  might  cause  the  blue  to  change  to  red.  This  con- 
dition has  not  been  taken  cognizance  of  up  to  the  present  time. 
Secondly,  the  reduction  may  be  complete  and  the  blue  color  be 
attributed  to  the  flocculent  union  of  particles  already  spoken  of;  or 
perhaps  to  the  irregular  growth  so  that,  instead  of  flakes  or  needles, 
husk-shaped  bodies  are  called  into  being.  These  of  course  may  be 

*  F.  Kirchner  und  R.  Zsigmondy:  c.  Drudes  Annalen  d.  Phys.  (4),  15,  573-595 
(1904);  R.  Zsigmondy:  Zur  Erkenntnis  der  KoUoide,  114  (1905). 

t  F.  Kirchner:  Ber.  d.  Kgl.  Sachs.  Ges.  d.  Wiss.  Leipzig,  54,  Math.  phys.  KL, 
261-266  (1902). 

t  M.  Planck:  Drudes  Annalen  d.  Phys.  (4),  1,  69-122  (1900);  Sitzungsber.  d. 
Klg.  Akad.  d.  Wiss.  zu  Berlin  (1902),  470-494. 

§  G.  Mie:  I.  c.  page  98. 

f  H.  Siedentopf:  Verb.  d.  Deutsch.  Phys.  Ges.,  12,  36  (1910). 

||  R.  Zsigmondy:  Zur  Erkenntnis  der  KoUoide,  114,  133-134  (1905). 


INORGANIC  COLLOIDS  103 

submicroscopic.  Finally,  the  liquid  may  contain  large  massive  gold 
particles  that,  according  to  the  theory  of  Mie,  would  account  for  the 
blue  color. 

The  Behavior  of  Colloidal  Gold  under  the  Influence  of  a  Fall 

of  Potential 

Under  the  influence  of  a  potential  fall  dialyzed  and  concentrated  gold 
solutions  behave  differently  from  freshly  prepared  hydrosols  that  con- 
tain electrolytes.  In  both  cases  the  particles  migrate  toward  the 
anode;  a  phenomenon  that  is  easily  demonstrated  in  Coehn's  ap- 
paratus, as  described  on  page  47.  If,  however,  the  electrodes  are 
dipped  into  non-dialyzed  gold  solutions  the  migration  is  not  easily 
seen.  Frequently  there  is  no  precipitation  on  the  anode,  or  at  least 
the  amount  is  inappreciable.  Instead,  most  of  the  gold  sinks  to  the 
bottom  in  the  form  of  red  clouds,  and  occasionally  there  is  an  accom- 
panying change  of  color.  The  cause  for  this  lies  in  the  fact  that  acids 
or  free  halogens  are  generated  at  the  anode,  the  charge  on  the  parti- 
cles is  changed,  and  these  start  off  toward  the  cathode.*  Some  of  the 
particles  are  now  charged  positively  while  others  are  still  negative. 
The  two  charges  neutralize  each  other,  and  the  particles  unite  to  form 
electrically  neutral  combinations  that  fall  out  of  solution.  If  chlorine 
is  evolved  at  the  anode  it  partially  dissolves  the  gold  to  form  gold 
chloride. 

With  well  dialyzed  or  concentrated  solutions  a  precipitation  of  the 
gold  on  the  anode  in  the  form  of  a  black  powder  may  be  obtained.! 
This  black  powder  after  it  has  been  dried  has  the  glance  of  gold. 
Here  traces  of  impurities  may  act  as  protective  colloids,  and  no  pre- 
cipitation occurs  on  the  anode,  but  instead  a  concentration  in  the  anode 
portion  may  take  place.  The  author  was  enabled  to  observe  such  a 
condition  of  affairs  during  the  electrolysis  of  gold  solutions  containing 
gelatin. 

Galecki  t  has  recently  carried  out  elaborate  investigations  on  the 
electrical  migration  of  gold  particles.  He  found  in  confirmation  of  the 
work  of  Blake  and  Svedberg  §  that  increased  additions  of  electrolytes 
gradually  discharged  the  particles,  and  that  the  system  became  un- 
stable in  the  region  of  the  isoelectric  point.  He  found  further  that  the 
mobility  of  the  particles  under  a  given  potential  difference  was  in- 

*  J.  Billitzer:  Zeit.  f.  phys.  Chemie,  45,  307-330  (1903).  J.  C.  Blake:  Zeit.  f. 
anor.  Chemie,  39,  72-83  (1904). 

t  R.  Zsigmondy:  Liebigs  Annalen,  301,  36  (1898). 

J  A.  v.  Galecki:  Zeit.  f.  anorg.  Chemie,  74,  174-206  (1912). 

§  Chapter  III,  page  49. 


104  CHEMISTRY  OF  COLLOIDS 

dependent  of  their  size,  but  appreciably  affected  by  the  accidental 
presence  of  electrolytes.  In  general  the  mobility  increases  after  dialy- 
sis. The  absolute  rate  of  migration  of  the  particles  in  a  well  dialyzed 
hydrosol  may  reach  that  of  the  chloride  ion. 

Reactions  of  Colloidal  Gold 

The  gold  in  hydrosols  amalgamates  very  incompletely  or  not  at  all 
with  mercury.  A  well  dialyzed  or  concentrated  gold  solution  suffers  no 
perceptible  change  after  two  or  three  days'  shaking  with  mercury.  If 
the  experiment  is  kept  up  for  several  weeks  with  frequent  shaking 
there  is  often  in  specific  cases  a  change  in  the  shade  of  the  solution,  and 
an  increase  in  turbidity  may  be  noted.  The  cause  of  this  is  doubtless 
the  dissolution  of  the  mercury  to  form  an  electrolyte,  and  the  con- 
sequent partial  coagulation  of  the  hydrosol.  This  phenomenon  has  not 
been  studied  with  the  ultramicroscope.  Precipitated  colloidal  gold  and 
the  purple  of  Cassius  behave  similarly.  It  should  be  noted  that  Ber- 
zelius  took  the  fact  that  the  purple  of  Cassius  would  not  unite  with 
mercury  to  indicate  that  the  purple  was  a  combination  of  gold  oxide 
with  tin  oxide. 

On  account  of  the  negative  charge  the  gold  particles  are  particu- 
larly sensitive  to  the  precipitating  action  of  cathions.  Because  trivalent 
ions  have  a  more  intense  effect  than  bivalent,  and  the  bivalent  greater 
than  univalent,  extraordinarily  small  amounts  of  trivalent  cathions 
precipitate  gold  from  colloidal  solutions.  Excess  of  aluminium  salts  may 
cause  a  reversal  of  sign  and  form  a  stable  positively  charged  hydrosol. 
Dyestuffs,  such  as  Fuchsin,  Bismarck  Brown,  etc.,  that  migrate  toward 
the  cathode,  precipitate  gold  with  the  accompanying  change  of  color 
and  are  carried  down  with  it.  After  the  precipitate  has  settled  out  the 
liquid  is  often  colorless.  The  dyestuff  cannot  be  dissolved  out  by 
water  but  can  be  extracted  with  alcohol.  The  residue  is  in  the  form 
of  a  black  powder.  Likewise,  positively  charged  colloids,  such  as  iron 
oxide,  aluminium  hydroxide,  zirconium  oxide,  etc.,  precipitate  the  gold 
as  has  been  shown  by  Biltz.  Definite  quantity  relations  must  be 
maintained,  however,  in  order  to  precipitate  completely  both  colloids. 
Table  15  *  represents  these  relations  for  colloidal  gold  and  iron  oxide 
solutions. 

Similar  relations  hold  for  gold  fuchsin  precipitation;  an  excess  of 
either  leaves  the  liquid  colored.  The  position  of  the  optimum  depends 
upon  the  concentration  of  both  solutions,  upon  the  state  of  subdivision 
of  the  gold,  and  upon  some  other  factors. 

*  W.  Biltz:  Ber,  37,  1104  (1904). 


INORGANIC  COLLOIDS 
TABLE  15 


105 


10  cc.  of  gold  solution  containing  1.4  mg.  Au  were  mixed  with  5  cc.  of  iron  oxide 
solution,  the  concentration  of  which  varied. 


Fe2O3  mg. 


Observations. 


Immediate. 


After  one  hour. 


8.0 
4.0 

3.2 

2.4 
1.6 
0.8 

0.32 


No  precipitation 

Formation  of  flocks,   very  slow  I 

settling 
Complete  precipitation 

Large  flocks,  slow  precipitation 
Flocksj  solution  red 
Fine  clouds,  very  slow  precipita- 
tion 
No  precipitation 


Large  flocks  settled  slowly 
Same  as  above 

Complete  precipitation,  liquid 

colorless 
Same  as  above 
Flocks,  solution  rose  color 
Flocks,  solution  rose  color 

Very  slight  turbidity 


Adsorption  of  Colloidal  Gold  by  Aluminium  Hydroxide,  Fibers, 
and  Barium  Sulfate 

Of  interest  is  the  adsorption  of  colloidal  gold  by  substances  such  as 
dissolved  dyestuffs.  This  property  depends  less  upon  the  surface  of 
the  metal  than  it  does  upon  the  degree  of  subdivision  of  the  gold,  that 
is  upon  the  number  and  size  of  the  particles.  A  typical  case  is  the 
adsorption  of  gold  by  aluminium  hydroxide.  When  the  hydroxide  and 
the  gold  solution  are  shaken  together  the  former  becomes  more  or  less 
intensely  red,  just  as  it  does  when  shaken  with  Carmine  Red.  In  both 
cases  under  suitable  circumstances  the  liquid  is  rendered  colorless.  In 
the  one  case  a  carmine  lacquer,  and  in  the  other  a  lacquer  like  combi- 
nation of  gold  and  hydroxide  is  obtained.  The  formation  of  the  gold 
lacquer  can  be  followed  under  the  ultramicroscope.  Gold  solutions 
and  fibers  affect  each  other  in  a  similar  manner.* 

Gold  and  Barium  Sulfate.  —  A  number  of  fine  crystalline  precipi- 
tates, such  as  calcium  carbonate,  strontium  carbonate,  and  barium 
sulfate,  also  have  the  property  of  adsorbing  gold  particles.  The  last- 
named  reaction  was  discovered  by  Vanino,f  who  showed  that  barium 
sulfate  may  be  used  in  this  way  with  a  number  of  irreversible  hydro- 
sols.  In  an  account  of  an  investigation  not  published  he  demon- 
strated that  only  the  smaller  particles  of  the  barium  sulfate  have  the 
property  of  uniting  with  the  gold.  The  amount  of  sulfate  necessary 
to  remove  the  color  from  5  cc.  of  the  gold  solution  depends  upon  quality 

*  R.  Zsigmondy:  Verb.  d.  Ges.  D.  Naturf.  u.  Artze,  73  Vers.,  171.  Hamburg 
(1901).  W.  Biltz:  Nachr.  d.  Kgl.  Ges.  d.  Wiss.  zu  Gottingen,  Math.-phys.  Kl., 
18-32  (1904);  Ber.,  37,  1095-1116  (1904). 

t  L.  Vanino:  Ber.,  35,  662  (1902). 


106  CHEMISTRY  OF  COLLOIDS 

of  the  barium  sulfate.  Of  a  preparation  purchased  21  to  60  mgs. 
were  necessary  to  decolorize  completely  5  cc.  of  a  gold  solution  con- 
taining 0.25  mg.  of  gold.  After  the  addition  of  a  sufficient  quantity 
of  protective  colloid  no  decolorization  by  the  sulfate  could  be  observed. 
There  is  a  lower  concentration  limit  of  the  protective  colloid  under 
which  the  precipitation  is  not  prevented.  In  the  case  in  point  the 
lower  limit  with  5  cc.  of  the  above  gold  solution  and  60  mgs.  of  barium 
sulfate  is  as  follows: 

Glue 0. 1  mg. 

Gum  arabic 0.044 

Albumin 0 . 5  to  1 . 5 

If  less  than  these  amounts  of  protective  colloids  are  added  the  gold  is 
completely  coagulated.  On  the  other  hand  an 'excess  produced  a  white 
precipitate  leaving  a  red  liquid  that  contained  the  gold  and  the  pro- 
tective colloid.  When  both  the  sulfate  and  the  protective  colloid  are 
increased  complications  arise.  There  may  be  a  partial  coagulation  of 
both  the  gold  and  the  protective  colloid,  or  both  may  be  adsorbed  by 
the  sulfate. 

Protective  Effect  and  the  Gold  Number 

The  sharp  change  of  color  makes  gold  solutions  particularly  well 
adapted  for  demonstrating  the  effect  of  protective  colloids.  These 
solutions  served  to  demonstrate  for  the  first  time  that  protection  was  a 
general  property  of  reversible  and  also  some  irreversible  colloids,  and 
that  chemical  reactions  were  not  necessarily  involved.*  The  change 
of  color  by  electrolytes  in  red  gold  solutions  is  prevented  by  protective 
colloids,  but  the  specific  members  of  the  class  differ  markedly  in  their 
effect.  We  have  at  hand,  therefore,  a  means  of  further  characterizing 
this  group  of  substances.  This  is  done  by  means  of  the  gold  number. 

By  the  gold  number  we  will  understand  the  maximum  number  of 
milligrams  of  protective  colloid  that  may  be  added  to  10  cc.  of  gold 
solution  without  preventing  a  change  of  color  from  deep  red  to  violet 
shades  by  1  cc.  of  a  10  per  cent  solution  of  sodium  chloride,  where  the 
change  would  take  place  if  no  protective  colloid  were  added. 

For  the  determination  of  the  gold  number  hydrosols  prepared  by 
the  formaldehyde  method  having  particles  lying  between  20  and  30  w 
are  the  most  suitable.  The  correct  degree  of  subdivision  may  be 
known  by  a  faint  brownish  opalescence  in  incident  light;  in  trans- 
mitted light  the  solution  must  be  deep  red  and  clear.  If  the  pro- 
tective effect  of  the  colloid  in  question  is  approximately  known,  it  is 
wise  to  dilute  until  a  few  tenths  of  a  cubic  centimeter  will  prevent  the 
*  R.  Zsigmondy:  Zeit.  f.  analyt.  Chemie,  40,  697-719  (1901). 


INORGANIC  COLLOIDS 


107 


color  change.    If  the  effect  is  quite  unknown  it  should  be  roughly 
determined  before  accurate  measurements  are  attempted. 

0.01,  0.1,  and  1  cc.  of  the  solution  to  be  determined  (a,  b,  and  c)  are 
put  into  three  small  beakers  and  thoroughly  mixed  with  10  cc.  of  gold 
solution.  At  the  end  of  three  minutes  1  cc.  of  a  10  per  cent  sodium 
chloride  is  added  to  each  and  the  contents  well  mixed.  Assuming  that 
there  is  a  color  change  in  (a)  but  not  in  (6)  nor  (c),  the  gold  number 
must  lie  between  0.1  and  0.01.  For  more  accurate  determinations 
0.02,  0.05,  and  0.07  cc.  of  the  protective  colloid  should  be  taken  and  the 
procedure  repeated.  Where  the  color  is  not  sharp  interpolation  be- 
tween unquestioned  concentrations  must  be  resorted  to.*  From  the 
following  table  it  will  be  gathered  that  the  gold  number  varies  greatly, 
and  can  therefore  be  used  to  characterize  this  class  of  substances. 

TABLE  16 


Colloid. 

Gold  number. 

Reciprocal  gold 
number. 

Class  of 
protective 
colloid. 

Gelatin  and  glues 

0  005-0  01 

200-100 

Isinglass 

0.01-0.02 

100-50 

I 

Casein      .                                      .    . 

0  01 

100 

Gum  arabic,  good.                      

0.15-0.25 

6.7-4 

Gum  arabic,  poor  ....           

0.5-0.4 

2-0.25 

II 

Sodium  oleate  

0.4-1 

2.5-1 

Tragacanth  

2  (about) 

0.5  (about) 

Dextrin  

f    6-12 

0.17-0.08 

TTT 

Potato  starch 

\  10-20 
25  (about) 

0  .  1-0  .  05 
0  04  (about) 

Silicic  acid 

oo 

0 

Aged  stannic  acid 

GO 

0 

IV 

Slime  from  the  kernel  of  quince  .  .  . 

GO 

0 

In  the  table  the  reciprocal  gold  numbers  make  the  comparison  more 
striking,  The  results  are  reproducible  provided  the  same  gold  solu- 
tion is  employed  under  the  same  conditions.  For  this  reason  it  is 
possible  to  detect  a  change  in  the  gold  solution  by  this  method,  al- 
though the  changes  of  state  in  the  gold  solutions  do  not  have  so  great 
an  influence  as  the  differences  in  the  quality  of  the  protective  colloid.t 
An  example  of  how  the  gold  number  may  be  employed  to  distinguish 
between  protective  colloids  was  worked  out  jointly  with  Schulz  in 
Jena.  The  white  of  a  hen's  egg  may  be  separated  into  different  portions 
by  fractional  precipitation  with  ammonium  sulfate.t  The  individual 

*  R.  Zsigmondy:  Zeit.  f.  analyt.  Chemie,  40,  697-719  (1901). 
t  Fr.  N.  Schulz  und  R.  Zsigmondy:   Hofmeisters  Beitrage  z.  chem.  Physiol.  u. 
Pathol.,  3,  137-160  (1902). 
Ibid. 


108  CHEMISTRY  OF  COLLOIDS 

fractions  are  not  distinct  chemical  bodies,  nevertheless  they  have  certain 
characteristics. 

The  first  fraction  contains  globulin,  then  follows  crystallized  albu- 
min and  finally  amorphous  albumins  mixed  with  ovomucoids.  Ovomu- 
coid  has  the  peculiar  property  of  not  coagulating  when  the  slightly 
acid  egg  solution  is  boiled,  and  can  therefore  be  isolated.  Globulins  are 
insoluble  in  pure  water  but  require  a  certain  amount  of  salt  before 
they  will  go  into  solution.  The  gold  number  of  globulin  lies  between 
0.02  and  0.05  while  that  of  the  ovomucoids  lies  between  0.04  and  0.08. 

The  individual  albumin  fractions  differ  very  greatly.  The  first 
fraction  obtained  by  the  presence  of  |  per  cent  sulfuric  acid  separates 
out  in  the  form  of  microscopic  crystals,  some  of  which  are  well  defined. 
After  several  recrystallizations  the  gold  number  remained  constant  be- 
tween 2  and  8.  In  order  to  obtain  these  constant  gold  numbers  it- 
is  necessary  to  recrystallize  more  often  than  is  usual  in  physiological 
chemistry.  The  next  albumin  fraction  is  amorphous  and  has  the 
striking  property  of  turning  the  gold  solution  turbid  and  blue  or  violet 
without  the  aid  of  an  electrolyte.  The  third  fraction  is  likewise  amor- 
phous and  in  contradistinction  to  the  second  fraction  exercises  a  high 
degree  of  gold  protection.  The  gold  number  was  determined  in  the 
normal  way  and  lay  between  0.03  and  0.06.  Although  the  three  frac- 
tions of  albumin  had  similar  chemical  properties,  yet  they  showed 
marked  differences  toward  gold  solutions. 

Zunz  *  has  made  similar  observations  in  the  case  of  albumoses. 
As  is  well  known  albumoses  are  decomposition  products  of  egg  albumin 
and  are  produced  during  digestion.  They  are  more  closely  related  to 
albumin  than  are  the  peptones,  and  manifest  great  differences  in  diffu- 
sion in  that  primary  albumoses  diffuse  with  difficulty  while  the  second- 
ary albumoses  diffuse  easily.  The  primary  albumoses  are  more  easily 
precipitated  by  ammonium  sulfate  than  are  the  secondary.  To  the 
primary  albumoses  belong  heteroalbumoses  soluble  only  in  dilute  salt 
solutions,  protalbumoses  soluble  in  pure  water,  and  synalbumoses. 
The  gold  numbers  found  by  Zunz  *  for  the  three  decomposition  prod- 
ucts are  as  follows: 

Heteroalbumoses 0.01  to  0.075 

Protalbumoses 1.6    to  3.36 

Synalbumoses  give  violet  color  at 0 . 64  to  2 . 24 

The  synalbumoses  have  no  protective  action  on  gold  solutions  but  act 
in  a  manner  very  similar  to  the  second  albumin  fraction.    Zunzf  has  re- 

*  E.  Zunz:  Archives  internal,  de  Physiol.,  1,  427-439  (1904). 

t  E.  Zunz:  Bull.  Soc.  Roy.  des  Sc.  med.  et  nat.,  64,  174-186  (1906). 


INORGANIC  COLLOIDS 


109 


cently  investigated  the  behavior  of  a  series  of  secondary  peptones  and 
albumoses  toward  gold  solutions  and  found  that  all  have  the  property  of 
turning  the  color  from  red  to  blue,  but  that  they  possess  this  property 
in  a  varying  degree.  The  principal  results  are  set  forth  in  Table  17. 

TABLE  1 


Substance. 

Smallest  amount  of 
substance  in  mgs.  that 
will  suffice  to  turn  10 
cc.  colloidal  gold 
solution  violet. 

Substance. 

Smallest  amount  of 
substance  in  mgs.  that 
will  suffice  to  turn  10 
cc.  colloidal  gold 
solution  violet. 

Thioalbumoses  

2.60-4.00 

Albumoses  B  

0.70-1.80 

Albumoses  A  
B.. 
"          B 

2.24-3.20 
0.08-0.32 
0  20-0  80 

B  
C  

Peptone 

0.80-2.80 
1.60-3.20 
0  24-0  52 

"          B... 

0  50-1  40 

it 

3  60-7  40 

"          B 

0  40-1  20 

« 

4  40-8  20 

"          B  

0  80-1  60 

Zunz  *  found  further  that  there  is  no  relation  between  the  effect  of 
albumoses  and  peptones  on  gold  solutions  and  the  change  of  the  surface 
tension  of  the  water  through  these  substances. 

TABLE  18 


Substance. 

Concentration. 

Gold  number. 

Number  of  drops  in 
stalagmometer,  100 
drops  distilled  water 
taken  as  standard. 

Heteroalbumoses  

Per  cent. 
0   1 

0  01-0  075 

114  4 

Protalbumoses  

0.1 

1  6-3  36 

113  0 

Syn  albumoses 

01    { 

Causes  color 

I           113  7 

Egg  albumin  

\ 

1.0 

change 
About  0  1-0  3 

105  5 

From  the  table  it  will  be  seen  that  albumin  changes  the  surface  tension 
very  little,  while  the  albumoses  cause  a  very  appreciable  change.  Again 
the  albumoses  vary  enormously  in  their  effect  on  gold  solutions,  while 
their  change  of  surface  tension  of  the  water  is  almost  identical,  as  seen 
from  the  stalagmometrical  measurements. 

In  a  later  article  Zunz  f  has  pointed  out  that  there  is  no  relation  be- 
tween the  effect  of  the  albumoses  on  mastic  solutions  and  the  effect  on 
gold  solutions.  Heteroalbumoses  and  synalbumoses  precipitate  mastic 
turbidities,  while  other  albumoses  do  not.  On  the  other  hand  hetero- 
albumoses  have  a  protective  action  on  gold  solutions. 

*  E.  Zunz:  Bull.  Soc.  Roy.  des  Sc.  med.  et  nat.,  64,  187-203  (1906). 
t  E.  Zunz:  Arch,  internat.  de  Physiol.,  6,  111,  245-256  (1907). 


110 


CHEMISTRY  OF  COLLOIDS 


From  what  has  been  said  it  will  be  seen  that  the  electric  charge  on 
the  particles  is  not  the  only  factor  involved.  For  if  this  were  the  case 
both  gold  and  mastic  solutions  should  be  affected  in  a  similar  manner, 
as  they  are  both  negative.  Rather  must  the  action  be  attributed  to 
specific  properties  that  have  not  yet  been  clearly  explained. 

The  gold  number  has  also  been  employed  to  determine  whether  or 
not  changes  have  taken  place  in  protective  colloids  as  a  result  of  stand- 
ing or  of  temperature  variations.  It  has  been  demonstrated  that  the 
protective  property  decreases  in  intensity  with  the  lapse  of  time.  This 
decrease  may  be  caused  by  partial  coagulation,  as  well  as  by  chemical 
changes  such  as  hydrolysis,  decomposition,  etc.  A  number  of  colloids 
possess  a  different  gold  number  at  boiling  point  from  that  at  the  tem- 
perature of  the  room. 

A  systematic  investigation  of  the  change  of  state  in  gelatin  solutions 
has  been  carried  on  by  Menz  *  at  the  instigation  of  the  author.  Con- 
centrated gelatin  solutions  harden  to  jellies  whereby  numerous  large 
submicrons  are  formed.  In  more  dilute  solution  the  same  changes  take 
place  but  the  ultramicrons  formed  are  smaller  the  more  dilute  the 
solution.  It  can  be  easily  seen  from  the  following  table  that  the  pro- 
tective effect  is  greater  the  more  dilute  the  solution;  that  is  the  gold 
number  is  smaller  the  smaller  the  ultramicrons  are. 

TABLE  19 


Concentration  of  gela- 
tin solutions  in  the 
cold. 

Gold  number. 

One  day  old. 

Constant  some  days 
later. 

Per  cent. 
1 
0.5 
0.1 
0.01 
0.001 

0.037 
0.023  . 
0.015 
0.014 
0.0065 

0.039 

0.025 
0.016 
0.015 
0.012 

The  gelatin  particles  in  a  1  per  cent  solution  are  so  large  that  they 
may  be  seen  with  the  ultramicroscope.  It  is  remarkable  that  the  large 
particles  as  well  as  the  small  ultramicrons  unite  with  the  gold,  and 
that  the  reaction  may  be  followed  under  the  ultramicroscope.  Con- 
trary to  the  action  of  the  small  particles,  where  probably  several  of 
the  gelatin  particles  unite  with  one  gold  ultramicron,  the  large  gelatin 
particles  take  up  many  gold  particles.  This  happens  at  the  same  time 
as  a  gradual  change  of  color  toward  the  purple  and  an  increase  in  the 
turbidity  of  the  gold  gelatin  mixture.  Should  the  concentration  of  the 
*  W.  Menz:  Zeit.  f.  phys.  Chemie,  66,  129-137  (1909). 


INORGANIC  COLLOIDS  111 

gelatin  be  high  enough  the  gold  gelatin  clouds  fall  to  the  bottom.  In 
spite  of  this  the  gelatin  has  had  a  protective  effect  and  sodium  chloride 
causes  no  change  of  color  toward  the  blue  if  sufficient  gelatin  is  present. 

It  is  not  uninteresting  that  protective  colloids  are  present  in  normal 
urine,  and  that  these  behave  much  as  gelatin  does.  That  is  to  say  the 
degree  of  subdivision  increases  on  boiling,  whereby  the  protective 
effect  is  increased.  These  relations  were  discovered  by  Lichtwitz  and 
were  studied  by  him  in  collaboration  with  Rosenbach.*  These  col- 
loids in  urine  usually  escape  observation  because  their  protective  effect 
is  overshadowed  by  the  precipitating  influence  of  the  dissolved  electro- 
lytes. By  dialysis,  shaking  out  with  benzine,  or  by  precipitating  the 
protective  colloid  with  alcohol  according  to  Salkowsky,f  the  colloid 
may  be  separated  from  the  most  of  the  electrolyte,  when  the  protective 
effect  may  be  demonstrated.  The  gold  number  of  these  bodies  lies 
between  0.3  and  1.2.  It  is  a  noteworthy  fact  that  the  increased  pro- 
tective action  of  the  urine  colloids  is  associated  with  the  property  of  the 
urine  to  prevent  the  precipitation,  on  cooling,  of  the  uric  acid  sediment 
formed  by  the  boiling.  Such  samples  of  urine,  the  gold  number  of  which 
has  not  changed  on  boiling,  allow  the  uric  acid  sediment  to  precipitate 
immediately  on  cooling;  while  those  the  protective  action  of  which  is 
increased  from  1  to  10,  or  1  to  5,  prevent  the  precipitation  wholly  or 
at  least  partially. 

Of  general  interest  is  the  observation  that  urea,  uric  acid,  urochrome, 
hippuric  acid,  and  hypoxanthin  have  no  protective  action,  while  nu- 
cleinic  acid  is  a  protective  colloid  having  a  gold  number  2.5.  Urine 
albumin  does  not  always  operate  as  a  protective  colloid.  J  The  cause  is 
probably  to  be  sought  for  in  the  size  of  the  particles,  which  latter  may 
be  seen  under  the  ultramicroscope  as  established  by  the  work  of  Raehl- 
mann,  Much,  Romer,  and  Siebert.§ 

Theory  of  the  Protective  Action 

On  the  basis  of  an  elaborate  investigation  the  author  If  has  expressed 
the  opinion  that  the  protective  action  is  due  to  the  union  of  several 
ultramicrons  of  the  protective  colloid  with  a  particle  of  the  gold;  or 
vice  versa,  the  gold  particles  are  adsorbed  by  a  protective  individual. 

*  L.  Lichtwitz  und  O.  Rosenbach:  Hoppe-Seylers  Zeit.  f.  physiol.  Chemie,  61, 
112-118  (1909).  L.  Lichtwitz:  Ibid.,  64,  144-157  (1910).  O.  Rosenbach:  Inaug.- 
Diss.  Gottingen  (1909). 

t  Salkowsky:  Berl.  klin.  Wochenschr.,  Nr.  51-52  (1905). 

j  L.  Lichtwitz:  Hoppe-Seylers  Zeit.  f.  physiol.  Chemie,  72,  215-225  (1911). 

§  E.  Raehlmann:  Munch,  med.  Wochenschr.,  Nr.  48  (1903).  H.  Much,  P. 
Roemer  und  C.  Siebert:  Zeit.  f.  diat.  u.  phys.  Therapie,  8,  19-27,  94-96  (1904-5). 

IT  R.  Zsigmondy:  Verh.  d.  Ges.  D.  Naturf.  u.  Artze.,  168-172.     Hamburg  (1901). 


112  CHEMISTRY  OF  COLLOIDS 

Several  other  hypotheses  have  been  made.  For  instance  Bechhold,* 
Neisser  and  Friedemann  f  have  attributed  the  prevention  of  the  precipi- 
tation of  bacteria  and  suspensions  by  protective  colloids  to  a  homo- 
geneous encircling  of  the  suspended  particle.  While  this  explanation 
may  be  accepted  without  hesitation  for  the  case  of  coarser  suspensions, 
it  is  not  satisfactory  for  protected  ultramicrons  whose  dimensions  are 
in  the  neighborhood  of  the  molecular.  The  individual  particles  of  the 
protective  colloid  must  have  a.  certain  size,  and  the  protection  is  greater 
the  smaller  the  protected  ultramicrons.  In  order  that  a  particle  of 
gold  might  be  completely  surrounded  by  a  larger  gelatin  particle,  the 
latter  would  of  necessity  have  liquid  properties.  This  assumption  is 
not  borne  out  by  any  evidence  and  is  doubtful  in  the  extreme.  See 
Chapter  XII  on  the  ultramicroscopy  of  gelatin. 

Neisser  and  Friedemann  also  believed  that  only  oppositely  charged 
particles  could  have  a  protective  action.  This  point  of  view  is  also 
contrary  to  fact,  for  particles  of  like  charge  often  have  a  much  greater 
protective  action  than  those  of  opposite  charge.  Because  of  this  fact 
Billitzer  t  concluded  that  the  particles  of  the  protected  and  the  pro- 
tective colloid  do  not  unite  at  all,  but  that  the  protective  colloid  adsorbs 
the  precipitating  electrolytes.  This  is  also  untenable  because  a  good 
protective  colloid  will  offset  the  action  of  a  million  times  its  own  weight 
of  a  precipitating  electrolyte.  It  seems  much  more  in  accordance  with 
the  fact  to  assume  a  mutual  adsorption  as  already  explained  in  the 
author's  hypothesis  in  the  foregoing  paragraph.  As  early  as  1900  the 
author  demonstrated  that  gold  foil  adsorbed  gelatin  and  covered  itself 
with  a  layer  that  could  not  be  removed  by  boiling. water.  This  layer 
prevented  the  amalgamation  of  the  gold  with  mercury.  The  experi- 
ment turns  out  better  if  the  surface  of  the  gold  foil  is  roughened  by 
repeated  amalgamation  and  redissolving  of  the  mercury.  * 

Direct  evidence  for  the  union  of  the  particles  was  obtained  by  noting 
the  dependence  of  the  protective  effect  upon  the  concentration  and 
the  time  occupied  by  the  reaction.  If  the  protective  effect  is  not  due 
to  the  mutual  action  of  the  particles  of  the  two  colloids  upon  each 
other,  then  it  should  be  practically  instantaneous  as  soon  as  the  mixing 
is  complete.  Experiment  shows,  however,  that  several  minutes  are 
often  necessary  before  the  protection  is  complete.  §  The  time  is 
doubtless  taken  up  in  formation  of  the  union  between  the  particles. 
The  concentration  of  the  protective  colloid  when  it  is  added  to  the 

*  H.  Bechhold:  Zeit.  f.  phys.  Chemie,  48,  385-423  (1904). 

t  M.  Neisser  und  U.  Friedmann:  Munch,  med.  Wochenschr.,  61,  465-469,  827- 
831  (1903-4). 

J  J.  Billitzer:  Zeit.  f.  phys.  Chemie,  61,  129-166  (1905). 

§  R.  Zsigmondy:  Zeit.  f.  analyt.  Chemie,  40,  713  (1901). 


INORGANIC  COLLOIDS  113 


gold  solution  plays  an  important  part.  For  instance,  TMir  ing.  of 
gelatin  dissolved  in  3  cc.  water  have  a  protective  action  on  10  cc.  of 
gold  solution  if  let  stand  10  minutes  before  adding  more  water,  amount- 
ing to  20  cc.  If,  however,  the  20  cc.  are  added  to  the  gelatin  solution 
before  mixing  with  the  gold  solution  there  is  no  longer  a  protective 
action.  It  is  evident  from  this  that  it  is  not  the  presence  of  the 
gelatin  that  gives  the  protection,  but  the  action  of  the  gelatin  on  the 
gold  particles.  After  the  protective  .action  has  once  set  in  further 
dilution  does  not  counteract  it.  All  this  speaks  for  the  union  (ad- 
sorption) of  the  particles.  Moreover  the  union  of  the  particles  can  be 
followed  directly  in  the  ultramicroscope  when  the  particles  of  the  pro- 
tective colloid  are  large  enough.  Such  observations  have  been  made 
in  the  case  of  colloidal  aluminium  hydroxide  and  gold  solution,  and 
also  gelatin  and  gold.  When  the  particles  of  the  gelatin  are  large 
enough,  the  second  type  of  protective  action  is  manifested,  viz.,  the 
adsorption  of  several  gold  ultramicrons  by  a  particle  of  gelatin. 

The  mutual  adsorption  of  the  ultramicrons  results  in  the  gold  hydro- 
sol  losing  its  characteristic  properties,  viz.j  irreversible  dehydration 
and  sensitiveness  to  electrolytes,  while  the  complex  particles  assume 
the  properties  and  show  all  the  reactions  of  the  protective  colloid.  If 
the  protective  colloid  can  be  precipitated  by  a  given  reagent,  so  can  the 
gelatin  gold  complex.  On  the  contrary  if  the  protective  colloid  is 
unaffected  by  the  electrolyte  in  question,  the  complex  will  also  be  ex- 
empt from  the  influence. 

From  these  considerations  some  interesting  conclusions  may  be 
drawn  with  regard  to  the  nature  of  protective  colloids.  For,  if  the 
complex  particles,  consisting  of  one  large  gold  individual  and  several 
smaller  gelatin  ultramicrons,  have  the  same  properties  in  solution  as 
the  pure  gelatin,  then  these  properties  must  be  practically  independent 
of  the  size  of  the  particles,  or,  in  other  words,  of  the  state  of  subdivision. 
In  point  of  fact  the  larger  subdivisions  of  gelatin  have  almost  identical 
properties  with  the  finer.  On  the  other  hand  it  should  be  noted  that 
the  above  law  is  not  altogether  of  general  application.  There  are 
cases  where  the  gold  particles  do  not  entirely  lose  their  power  to  unite. 
In  such  instances  it  is  possible  to  separate  the  gold  from  the  excess  of 
the  protective  colloid  by  fractional  precipitation,  and  the  first  fraction 
then  usually  exhibits  reversible  properties.  Just  as  in  the  case  of  many 
chemical  reactions  the  addition  of  another  molecule  causes  a  complex 
formation  whose  properties  differ  from  those  of  the  original  molecules, 
so  in  this  case  also  two  different  bodies  unite,  and  the  resulting  com- 
plex has  properties  differing  from  either  of  the  original  components. 
Contrary  to  the  pure  chemical  reactions  we  do  not  have  to  do  here  with 


114  CHEMISTRY  OF  COLLOIDS 

new  bodies  of  constant  composition.  The  laws  of  definite  or  multiple 
proportions  are  not  involved,  and  the  complex  may  have  some  of  the 
properties  of  one  or  both  components.  It  is  important  to  note  that 
these  colloidal  complexes  exist  whose  reactions  may  imitate  those  of 
pure  chemical  substances,  because  this  has  led  to  many  misunder- 
standings and  wrong  conclusions.*  As  an  example  of  another  case 
may  be  cited  the  adsorption  of  colloids  by  fibers.  Here  again  the  re- 
actions so  nearly  resemble  those  truly  chemical  that  misconceptions  are 

liable  to  arise. 

B.  Colloidal  Platinum 

Colloidal  platinum  has  been  prepared  by  Lottermoser  f  in  a  chemical 
way  by  reduction  with  formaldehyde,  by  Gutbier  J  using  phenyl- 
hydrazin,  and  by  Bredig  §  by  means  of  electrical  colloidation.  When 
well  prepared  the  solution  is  a  brown,  faintly  turbid  liquid  containing 
sometimes  as  high  as  20  mgs.  of  platinum  per  100  cc.  This  hydrosol 
is  renowned  because  of  its  excellent  catalyzing  properties.  It  contains 
ultramicrons  differing  in  size  from  30  to  50  w,  which  refract  light  some- 
what less  than  gold  or  silver  particles.  They  are  generally  colored 
greyish  white  shading  into  yellow  or  blue.  They  possess,  in  a  high 
degree,  the  property  of  compact  platinum  to  catalyze  the  decomposition 
of  hydrogen  peroxide.  This  property  has  been  thoroughly  studied  by 
Bredig  and  his  students,  and  by  many  other  investigators.  The  reac- 
tion is  interesting  because,  as  Bredig  has  pointed  out,  there  is  remarkable 
analogy  between  it  and  that  of  blood  corpuscles,  enzymes,  and  fer- 
ments. For  this  reason  Bredig  has  called  his  hydrosols  "anorganische 
Fermente,"  although  colloidal  platinum  does  not  manifest  many  specific 
fermentation  effects.  Many  substances,  such  as  hydrocyanic  acid, 
hydrogen  sulfide,  etc.,  greatly  reduce  or  totally  destroy  the  catalytic 
effect  of  both  the  ferments  and  the  platinum  solutions.  Bredig,  in 
conjunction  with  his  collaborators, H"  has  carried  out  a  great  many 
series  of  experiments  on  the  catalytic  effect  of  platinum  hydrosols,  the 
most  important  results  of  which  are  given  below. 

Catalytic  Effect.  —  It  was  first  shown  that  exceedingly  small  amounts, 
one  gram  mol.  of  platinum  in  seven  million  liters  of  water,  served  to 
increase  perceptibly  the  rate  of  decomposition  of  hydrogen  peroxide. 

*  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  pages  56-61  (1905). 

t  A.  Lottermoser:  Anorganische  Kolloide,  33  (1901). 

}  A.  Gutbier:  Zeit.  f.  anorg.  Chemie,  32,  347-356  (1902). 

§  G.  Bredig:  Anorganische  Fermente,  30.  Leipzig  (1901).  Zeit.  f.  Elektro- 
chemie,  4,  514r-515  (1898). 

Tf  G.  Bredig:  Ibid.  Bredig  und  R.  Miiller  von  Berneck:  Zeit.  f.  phys.  Chemie, 
31,  258-353  (1899).  Derselbe  und  K.  Ikeda:  Ibid.,  37,  1-68  (1901).  Derselbe 
und  W.  Reinders:  Ibid.,  37,  323-341  (1901). 


INORGANIC  COLLOIDS 


115 


If  the  concentration  of  the  platinum  solution  is  decreased  in  geometri- 
cal proportion  from  2  to  1,  the  reaction  constant  decreases  from  3  to 
1.  The  reaction  is  of  the  first  order,  although  the  constant  ^decreases 
considerably  during  the  reaction.  The  rate  is  increased  by  small 
amounts  of  alkali,  but  large  amounts  retard  it.  It  is  noteworthy 
that  the  effect  of  alkalis  on  platinum  and  enzyme  solutions  is  very 
similar,  as  shown  by  Fig.  19. 

The  change  of  rate  between  25  and  85  degrees  was  determined  and 
the  temperature  coefficient  (for  10  degrees)  was  found  to  be  1.7  for  dilute 
acetic  acid  solution.  The  platinum  solution  must  not  be  warmed  too 
long  before  the  experiment,  especially  at  the  higher  temperatures,  else 
the  activity  of  the  catalyzer  is  lessened.  This  is  probably  due  to  the 
partial  coagulation  of  the  particles.  Warming  for  1J  hours  at  65  de- 
grees reduced  the  rate  in  the  relation  of  24  :  15.  Schoenbein  *  has 
observed  like  effects  by  the  warming  of  enzymes. 


500 


0.25         0.5 

Platinum 


nNaOH 

TSmulsion 

FIG.  19.    Effect  of  alkali  on  the  catalysis. 


.NaO-H 


Poison  Effects  on  Platinum  Sols.  —  Very  striking  is  the  singularity 
that  a  series  of  poisons  largely  prevent  or  completely  destroy  the  cata- 
lytic property  of  platinum  or  gold  hydrosols.  Mere  traces  of  hydro- 
cyanic acid,  hydrogen  sulfide,  arsenious  acid,  or  phosphorus  produce  an 
effect.  One  mol.  of  hydrogen  sulfide  in  ten  million  liters  of  water 
retarded  the  reaction  measurably.  Schoenbein  found  the  same  thing 
to  hold  for  aqueous  extractions  of  potato  peelings,  and  the  leaves  of 
Leontodon  Taraxacum.  The  activity  of  these  solutions  is  instantly  re- 
tarded by  hydrogen  sulfide.  Bredig  found  a  further  analogy  between 
platinum  and  ferment  catalysis  in  the  poisoning  by  hydrocyanic  acid. 
One  mol.  of  hydrocyanic  acid  in  20  million  liters  of  water  is  sufficient  to 
reduce  the  catalytic  action  to  one-half  its  former  value. 

*  C.  F.  Schoenbein:  Journ.  f.  prakt.  Chemie  (I),  89,  340  (1863). 


116  CHEMISTRY  OF  COLLOIDS 

Schoenbein  found  that  organic  materials  having  the  property  of  de- 
composing hydrogen  peroxide  lose  this  power  temporarily  when  treated 
with  hydrocyanic  acid.  He  suggested  that  the  poisonous  effect  of  this 
acid  on  the  blood  was  due  to  the  reduction  of  the  power  of  the  red 
corpuscles  to  activate  chemically  the  oxygen  of  the  air  breathed. 

Both  in  the  case  of  ferments  and  platinum  sols  the  catalytic  power 
is  revived,  although  somewhat  slowly.  The  cause  for  the  renewing  is 
probably  the  oxidation  of  the  poison  by  the  hydrogen  peroxide.  This 
latter  substance  is  a  well-known  antidote  for  hydrocyanic  acid  poison- 
ing in  the  human  body.  As  might  be  expected  from  these  consider- 
ations the  poisoning  effect  on  platinum  sols  is  much  less  if  hydrogen 
peroxide  is  present  when  the  poison  is  added.  This  is  also  true  for 
enzymes. 

Phosphorus  is  also  a  poison  for  platinum,  and  0.00004  of  a  mol.  is 
sufficient  to  retard  the  action  to  one-eighth  its  value.  The  relations  in 
the  case  of  carbon  monoxide  are  peculiar.  Shaking  platinum  with  this 
gas  stops  the  reaction  almost  entirely  in  about  one-half  hour.  Then  a 
recovery  sets  in  and  the  catalysis  is  more  vigorous  than  it  was  before 
the  poisoning.  On  the  other  hand,  if  hydrogen  peroxide  is  present  in 
excess  when  the  platinum  is  treated  with  the  carbon  monoxide  the 
catalysis  is  increased  from  the  beginning.  To  explain  this  remarkable 
phenomenon  it  has  been  suggested  that  the  carbon  monoxide  first 
poisons  it  and  then  further  subdivides  it  while  the  oxidation  of  the 
poison  is  going  on.  It  may  be,  however,  that  the  carbon  monoxide 
simply  removes  other  retarding  substances. 

Corrosive  sublimate  in  a  dilution  of  one  mol.  in  two  and  one-half 
million  liters  of  water  retards  the  catalysis  enormously,  and  the  plati- 
num does  not  regain  its  former  power.  Mercuric  cyanide  has  a  much 
weaker  retarding  action  on  both  bacteria  and  platinum  solutions.* 
The  far-reaching  analogy  between  the  poisoning  of  organic  materials 
and  platinum  solutions  makes  it  seem  probable  that  poisoning  effects 
are  due  to  the  retarding  of  catalytic  reactions  by  foreign  bodies. 

C.   Colloidal  Silver 

It  is  not  by  any  means  so  easy  to  obtain  colloidal  silver  free  from 
electrolytes  as  it  is  platinum  and  gold.  Colloidal  silver  without  the 
presence  of  some  protective  colloid  is  usually  in  the  form  of  coarser 
subdivision.  In  most  cases  it  also  contains  some  silver  oxide. 

Bredig's  Colloidal  Silver.  —  Colloidal  silver  free  from  electrolytes 
can  be  best  made  by  Bredig's  f  method  of  electrical  colloidation.  The 

*  Th.  Paul  und  B.  Kroenig:  Zeit.  f.  phys.  Chemie,  21,  414-450  (1896). 
f  G.  Bredig:  Anorganische  Fermente,  31.     Leipzig  (1901). 


INORGANIC  COLLOIDS  117 

hydrosols  obtained  are  usually  somewhat  turbid,  and  have  a  grey  or 
reddish  color.  Under  the  ultramicroscope  these  solutions,  as  well  as 
thoce  made  by  other  methods,  are  very  beautiful.  Bluish  red,  purplish, 
or  violet  stars  may  be  seen  moving  about  with  great  rapidity  in  the 
liquid.  When  only  one  sort  of  colored  submicrons  is  present,  blue,  for 
instance,  the  liquid  in  transmitted  light  has  almost  the  complementary 
color,  that  is  yellow  or  brown.  Because  there  are  a  great  many  differ- 
ent colored  particles  in  the  hydrosols  prepared  by  Bredig's  method  the 
liquid  is  a  combination  of  all  these  and  appears  grey.  Frequently  one 
color  predominates  even  in  these  solutions  prepared  by  electrical  col- 
loidation.  Silver  solutions  containing  only  amicrons  so  small  that  the 
light  cone  can  scarcely  be  seen,  are  usually  intense  brown  in  reflected 
light. 

Colloidal  Silver  by  Kohlschiitter's  Method.  —  Kohlschutter  *  dis- 
covered another  very  good  method  for  preparing  colloidal  silver;  viz., 
the  reduction  of  silver  oxide  in  water  by  hydrogen.  At  a  temperature 
of  50  or  60  degrees  silver  oxide  is  reduced  with  the  simultaneous  for- 
mation of  a  mirror  and  a  hydrosol.  The  reduction  takes  place  on 
the  walls  of  the  vessel  and  not  in  the  liquid  itself.  Strange  to  relate 
the  color  of  the  sol  depends  upon  the  nature  of  the  walls  without 
there  being  any  question  of  the  solubility  of  the  glass,  as  far  as  can 
be  ascertained.  Walls  composed  of  soft  glass  or  quartz  give  yellowish 
brown  hydrosols  while  Jena  glass  gives  red,  reddish  brown,  violet,  or 
blue.  With  platinum  walls  no  sol  formation  takes  place,  but  instead 
crystalline  silver  separates  out.  The  platinum  becomes  charged  with 
hydrogen  and  this  replaces  the  silver  in  AgOH  just  as  an  ordinary 
metal  would.  It  is  a  remarkable  fact  that  quartz  and  ordinary  glass 
have  the  same  effect  while  Jena  glass  is  totally  diiferent.  To  show 
that  the  solubility  of  the  glass  was  not  the  vital  factor  Kohlschutter 
extracted  ordinary  ground  glass  with  water  and  used  this  solution  in 
conjunction  with  Jena  glass  to  reduce  the  silver  hydroxide.  The  re- 
sulting hydrosol  had  exactly  the  same  color  that  was  obtained  by  the 
use  of  Jena  glass  alone.  It  is  obvious  therefore  that  the  surface  of  the 
glass  has  a  vital  role  to  play  in  the  form  and  size  of  the  particles. 

Silver  hydrosols  may  be  purified  to  a  considerable  extent  by  treat- 
ment with  hydrogen  in  platinum  vessels.  The  conductivity  was  re- 
duced to  about  one-tenth  its  former  value,  and  had  at  the  end  about 
three  times  the  conductivity  of  the  water  employed  for  the  prepara- 
tion. K  =  4  to  8  •  10"6.  The  purified  sol  still  contained  some  silver 
hydroxide.  It  may  be  safely  assumed  that  the  adsorbed  hydroxyl  ion 
insures  the  stability  of  colloid  and  that  this  ion  accounts  for  the  charge 
*  V.  Kohlschutter:  Zeit.  f.  Elektrochemie,  14,  49-63  (1908). 


118 


CHEMISTRY  OF  COLLOIDS 


on  the  ultramicrons.  The  purification  with  hydrogen  is  very  useful, 
because  dialysis  frequently  results  in  coagulation.  It  is  noteworthy 
that  the  unpurified  sol  contains  considerable  adsorbed  silver  hydroxide, 
which  later  may  be  precipitated  with  the  colloid  by  potassium  nitrate. 
This  silver  hydroxide  can  scarcely  take  part  in  the  conductivity  be- 
cause that  of  the  unpurified  hydrosol  corresponds  to  the  amount  of 
oxide  remaining  after  the  precipitating  of  the  colloid.  Most  of  the 
silver  hydroxide  can  be  reduced  by  hydrogen  without  any  appreciable 
change  in  the  color  of  the  hydrosol.  The  amount  of  the  absorbed 
oxide  is  greater  in  the  brown  hydrosols  than  in  the  variegated,  and  it 
is  probable  that  the  total  silver  surface  is  greater  in  the  former. 

D.  Other  Colloidal  Metals 

All  metal  hydro-  and  even  organosols  may  be  prepared  by  a  method 
worked  out  by  Svedberg  *  in  which  the  colloidation  occurs  by  means 
of  sparks  from  an  induction  coil.  With  suitable  apparatus  and  care- 
fully purified  ethyl  ether-pentane,  etc.,  as  disperse  media  colloidal 
alkali  and  alkaline  metals  were  obtained.  Table  20  gives  a  com- 
parison between  the  colors  of  ether  sols  of  alkali  metals,  varying  in 
degree  of  dispersion,  and  the  vapors  of  the  metals  themselves. 

TABLE  20 


Metal. 

Color  of  the  ethylethersol. 

Color  of  the  vapor  of 
the  metal. 

Small  particles. 

Large  particles. 

Li 

Brown 
Purple  violet 
Blue 
Greenish  blue 
Blue  green 

Brown 
Blue 
Blue  green 
Greenish 
Greenish  blue 

Na  

Purple 
Blue  green 
Greenish  blue 

K  

Rb  

Cs  

The  absorption  maximum  of  colloidal  sodium  changes  from  yellowish 
green  to  red  during  coagulation,  just  as  in  the  case  of  gold.  The  sta- 
bility of  the  organosols  diminishes  from  sodium  to  caesium.  Svedberg 
has  also  investigated  the  preparation  and  the  stability  of  other  metal 
colloids  (Mg,  Cu,  Cd,  Hg,  etc.)  especially  in  isobutyl  alcohol. 

Protected  Metal  Colloids 

Every  pure  metal  colloid  may  be  transformed  into  a  reversible  col- 
loid by  the  addition  of  a  protective  colloid.  Generally  the  reduction  of 
the  metal  is  achieved  in  a  solution  containing  the  protective  colloid; 

*  The.  Svedberg:  Ber.,  38,  3616-3620  (1905).  Studien  zur  Lehre  von  den  kol- 
loiden  Losungen.  Upsala  (1907). 


INORGANIC  COLLOIDS  119 

or  the  reduction  is  allowed  to  take  place  under  such  circumstances 
that  a  protective  colloid  is  produced  simultaneously  with  the  reduction 
of  the  metal,  as  in  the  case  of  Lea's  colloidal  silver. 

Lea's  Colloidal  Silver.  —  In  1889  Carey  Lea  *  published  an  account  of 
his  observations  on  a  modification  of  metallic  silver  soluble  in  water. 
The  article  did  not  attract  much  attention  at  the  time  although  it  opened 
up  a  new  and  very  interesting  field.  It  was  indeed  remarkable  that  a 
metal  the  insolubility  of  which  had  been  known  for  ages  could  be  made 
in  a  form  soluble  in  water.  It  was  true,  of  course,  that  the  allotropic 
silver,  as  Lea  called  it,  could  not  be  prepared  pure,  but  contained  only 
97  to  98  per  cent  silver.  The  remainder  consisted  of  a  colloidal  combi- 
nation of  citric  acid  and  iron. 

Lea  produced  his  allotropic  silver  (A)  by  treating  in  the  cold  200  cc.  of 
a  10  per  cent  silver  nitrate  solution  with  a  mixture  consisting  of  200  cc. 
of  30  per  cent  ferrous  sulfate,  250  cc.  of  a  40  per  cent  sodium  citrate  solu- 
tion, and  50  cc.  of  a  10  per  cent  sodium  carbonate  solution.  The 
violet  precipitate  was  filtered  and  washed  in  water.  In  order  to  remove 
the  impurities  the  precipitation  was  repeated  a  number  of  times  with 
ammonium  nitrate.  Finally  the  solution  was  evaporated  and  a  mass 
with  a  metallic  luster  obtained  that  consisted  mostly  of  silver.  Lea 
found  that  the  metal  did  not  diffuse  through  membranes  and  that  it 
could  be  freed  by  this  means  from  electrolytes.  Because  of  these  prop- 
erties and  also  because  inorganic  colloids  were  considered  to  be  allo- 
tropic modifications  of  the  metals  in  question,  Lea  decided  that  his 
silver  must  also  be  an  allotropic  form.  Even  today  we  are  not  in  a 
position  to  deny  this  assertion.  Barus  and  Schneider  f  have  shown  that 
it  is  not  at  all  necessary  to  assume  allotropic  modifications  in  order  to 
explain  the  subdivision  in  water  or  the  behavior  of  the  colloid  toward 
electrolytes.  We  have  therefore  no  direct  evidence  that  the  metal  in 
the  colloidal  state  is  not  an  allotropic  modification;  but  the  assumption 
is  quite  unnecessary  and  perhaps  improbable.  On  the  other  hand 
recent  work  has  shown  that  allotropy  is  not  so  uncommon  as  it  was 
previously  supposed,  and  if  the  rule  proposed  by  W.  Oswald  holds,  that 
the  more  unstable  form  appears  first,  it  may  very  well  be  that  colloidal 
metals  contain,  or  are,  allotropic  modifications. 

The  experiments  of  Lea  were  repeated  by  Prange  J  a  year  later. 
He  varied  Lea's  method  for  the  preparation  and  found  that  good  silver 
hydrols  do  not  show  the  Tyndall  effect.  It  was  wrongly  concluded 

*  M.  Carey  Lea:  Amer.  Journ  of  Sc.  (3),  37,  476-491  (1889). 
t  C.  Barus  und  E.  A.  Schneider:  Zeit.  f.  phys.  Chemie,  8,  278-298  (1891). 
j  J.  A.  Prange:    Receuil  d.  travaux  chim.  des  Pays-Bas,  9,   121-133  (1890); 
J.  B.,  634  (1890). 


120 


CHEMISTRY  OF  COLLOIDS 


from  this  by  Stoeckl  and  Vanino  *  that  the  polarization  of  the  light  by 
the  colloidal  solution  was  circular.  Later  observations  have  shown 
that  the  polarization  is  always  completely  or  partially  linear  and  never 
circular  nor  elliptical.  Prange's  experiments  and  also  those  of  Lea  f 
merely  show  that  silver  hydrosols,  like  those  of  gold,  may  be  obtained 
in  a  form  that  is  optically  homogeneous.t 

Prange's  solutions  contained  about  0.4  gm.  silver  in  the  liter,  and 
were  extremely  sensitive  to  electrolytes.  Even  quartz  and  graphite 
coagulated  the  solutions,  during  which  there  was  a  considerable  evolu- 
tion of  heat. 

Schneider  §  purified  his  silver  hydrosols  according  to  the  method  of 
Lea  except  that  he  used  alcohol  to  precipitate  the  colloid.  He  also 
showed  1f  that  silver  organosols  such  as  alcosol  and  glycerosol  could  be 
prepared.  He  obtained  the  alcosol  by  dialyzing  the  hydrosol  with 
absolute  ethyl  alcohol,  and  also  by  precipitating  the  hydrosol  with 
somewhat  dilute  alcohol.  The  residue  was  then  partially  dried  on 
porous  plates  and  finally  dissolved  in  absolute  alcohol.  In  this  manner 
wine  red  and  also  chlorophyl  green  sols  may  be  prepared  that  show  in- 
teresting reactions  in  organic  media.  Schneider  ||  demonstrated  that 
nonelectrolytes  could  coagulate  alcosols  as  is  clearly  set  forth  in  the 

following  table. 

TABLE  21 

Effect  of  different  substances  added  to  a  0.3  per  cent  silver  alcosol 


Instantaneous  coagulation. 

No  coagulation. 

Isopropyl  alcohol 
Primary  and  secondary  butylalcohols 
Trimethyl  alcohol 
Heptyl  alcohol 
Octan,  formaldehyde 

Propyl  alcohol 
Isobutyl  alcohol 
Cetyl  alcohol  (in  alcoholic  solution) 
Glycerin 

Silver  hydrosols  prepared  by  Lea's  method  exhibit  peculiar  reactions 
that  deserve  further  elucidation.  Alkali  sulfates,  nitrates,  and  citrates 
precipitate  soluble  silver,  while  iron,  nickel,  and  magnesium  sulfates,  and 
barium  or  silver  nitrates,  throw  down  silver  that  is  insoluble  in  water. 
The  insoluble  form  may  sometimes  be  peptised  with  borax  and  am- 
monium sulfate,  but  these  reactions  are  not  always  reproducible,  and 
it  often  happens  that  the  repetition  gives  totally  unexpected  results. 

*  K.  Stoeckl  und  L.  Vanino:  Zeit.  f.  phys.  Chemie,  30,  98-112  (1899). 

t  M.  Carey  Lea:  Zeit.  f.  anorg.  Chemie,  7,  341  (1894). 

J  Sven  Oden.:  Zeit.  f.  phys.  Chem.,  78,  682-707  (1912). 

§  E.  A.  Schneider:  Ber.,  25,  1281-1284  (1892). 

IT  E.  A.  Schneider:  Ibid.,  1283  (1892). 

||  E.  A.  Schneider:  Zeit.  f.  anorg.  Chemie,  7,  339  (1894). 


INORGANIC  COLLOIDS  121 

Of  the  many  remaining  forms  of  silver  Lea's  golden  yellow  modifica- 
tion deserves  a  word.  It  is  obtained  by  mixing  two  solutions  a  and  6. 
a  contains  200  cc.  of  a  10  per  cent  silver  nitrate  solution,  200  cc,  of  a 
20  per  cent  Rochelle  salt  solution,  and  800  cc.  of  distilled  water,  b  con- 
tains 107  cc.  of  a  30  per  cent  ferrous  sulfate  solution,  200  cc.  of  a  20  per 
cent  solution  of  Rochelle  salt,  and  800  cc.  of  distilled  water.  The 
second  solution  must  be  prepared  immediately  before  use  and  is  added 
to  the  first  with  constant  stirring.  A  glittering  red  precipitate  falls 
out  that  gradually  becomes  black,  but  looks  like  bronze  on  the  filter. 
It  is  washed  and  then  spread  out  on  a  watch  glass  or  dish  to  dry.  The 
residue  forms  into  lumps  that  resemble  gold.  If  it  is  allowed  to  dry 
on  glazed  paper  the  residue  looks  like  gold  leaves.  On  glass  it  dries 
to  a  beautiful  gold  mirror.  If  the  washing  is  carried  on  too  long  the 
color  takes  on  a  bronze  tone.  The  preparation  is  almost  99  per  cent 
silver.  It  is  insoluble  in  water,  and  shows  a  number  of  very  interest- 
ing reactions.  Oxidizing  agents,  alkali  sulfates,  and  dilute  solutions 
of  potassium  ferricyanide  cause  beautiful  interference  colors.  Pressure 
changes  it  to  normal  silver  having  the  characteristic  silver  color.  When 
the  preparation  is  impure  the  color  is  sometimes  black.  Warming 
changes  it  into  light  colored  silver,  and  oxidizing  agents  then  fail 
to  produce  the  interference  colors.  Long  continued  treatment  with 
light,  electricity,  or  shaking  produce^  the  same  effect  as  pressure. 
Samples  of  gold-colored  silver  that  had  traveled  over  2000  miles  loose 
in  a  small  vessel  turned  to  white  silver  en  route,  while  other  portions 
of  the  same  sample  packed  tightly  with  cotton  remained  unchanged.* 
These  circumstances  as  well  as  the  microscopic  observations  of  Am- 
bronn  f  on  the  spontaneous  transition  of  silver  crystals  speak  very 
strongly  in  favor  of  the  existence  of  allotropic  modifications. 

Crystallization.  —  Lea  set  aside  some  of  his  unpurified  red  solution 
in  a  stoppered  bottle  for  some  weeks.  At  the  end  of  that  time  a  crys- 
talline precipitate  was  found  on  the  bottom  that  under  a  magnifying 
glass  appeared  to  be  short  black  needles  and  thin  prisms.  On  the 
addition  of  water  the  form  of  the  crystals  was  changed,  but  they  did 
not  dissolve.  When  the  mixture  was  dried  a  glittering  green  mass  re- 
mained. The  observation  of  Lea  is  of  interest  because  it  indicates 
that  the  larger  ultramicroscopic  particles  grow  at  the  expense  of  the 
smaller  until  macroscopic  crystals  are  formed.  The  change  of  the 
crystals  by  water  is  probably  due  to  enclosed  substances  soluble  in 
water.  This  behavior  resembles  to  a  certain  degree  that  of  crystal- 

*  M.  Carey  Lea:  Kolloides  Silber  und  die  Photohaloide.  Deutsch  von  Liippo 
Cramer,  100.  Dresden  (1908). 

t  H.  Ambronn:  Zeitschr.  f.  wiss.  Mikroskopie,  22,  349-355  (1905). 


122  CHEMISTRY  OF  COLLOIDS 

lized  egg  albumin  and  the  zeoliths  which  also  distend  in  pure  water. 
Furthermore  Ambronn*  has  succeeded  in  forming  exceedingly  thin 
microscopic  crystals  that  exhibit  a  pleochroism  similar  to  that  of  the 
silver  gelatin  complex.  These  crystals  sometimes  underwent  a  change 
by  which  isotropic  crystals  were  formed. 

The  growth  of  gold  or  silver  particles  in  a  suitable  reducing  mixture 
is  well  known,  but  it  has  not  yet  been  found  possible  to  cultivate  gold 
crystals  by  this  method.  The  tendency  of  silver  colloid  to  form  crystals 
is  much  more  marked,  and  is  probably  due  to  its  greater  solubility. 
In  a  silver  sol  prepared  according  to  the  method  of  Lea  and  left  stand- 
ing for  about  9  months  the  author  was  able  to  detect  a  number  of 
glittering  crystals  with  the  aid  of  the  microscope. f  They  were  mostly 
tiny  stars  with  three  or  six  rays  resembling  snowflakes,  and  had  a  diame- 
ter of  as  much  as  \  mm.  in  some  cases.  Contrary  to  the  experience  of 
Lea,  however,  these  crystals  did  not  distend  in  water.  A  spongy  mass 
of  silver  in  water  contained  an  enormous  number  of  tiny  crystals  after 
standing  for  several  months. 

Technical  Colloidal  Silver 

Technical  colloidal  silver,  such  as  Argentum  Crede,  used  for  medicinal 
purposes  was  formerly  prepared  by  a  method  similar  to  that  of  Lea. 
"Collargol,"  the  much  more  stable  preparation  at  present  procurable 
on  the  market,  contains  an  organic  protective  colloid.  Lottermoser 
and  v.  Meyer  J  investigated  these  technical  preparations  and  found 
that  electrolytes  precipitated  the  first  variety;  also  that  alkali  and 
alkaline  earth  salts  of  those  acids  forming  insoluble  silver  salts  precipi- 
tated the  silver  in  a  form  insoluble  in  water.  On  the  other  hand  alkali 
salts  of  those  acids  that  form  soluble  silver  salts  precipitated  the  silver  in 
a  soluble  form.  Chlorides  of  the  heavy  metals  changed  the  silver  to  silver 
chloride.  These  authors  also  found  to  their  surprise  that  glue,  albumin, 
rubber,  etc.,  prevented  the  precipitation  of  the  silver  by  the  electro- 
lyte. Today  we  know,  of  course,  that  this  is  a  general  property  of 
protective  colloids. 

Medical  Uses.  —  Colloidal  silver  is  used  for  intravenous  injection, 
or  it  is  made  into  a  salve  for  external  application  in  the  case  of  acute 
rheumatism,  pneumonia,  pysemia,  etc.  As  to  its  medicinal  effects  the 
medical  world  is  somewhat  divided.  Wolfrom  §  has  pointed  out  that 

*  I.e. 

t  P.  P.  von  Weimarn:  Koll.-Zeit.,  6,  62  (1909). 

j  A.  Lottermoser  und  E.  v.  Meyer:  Zeit.  f.  prakt.  Chemie  (2),  66,  241-247  (1897); 
67,  540-543  (1898). 

§  G.  Wolfrom:  Munch,  med.  Wochenschr.,  66,  1377-1382  (1909). 


INORGANIC  COLLOIDS  123 

the  varying  experiences  may  be  due  to  the  differences  in  the  prepa- 
rations employed.  Only  the  preparations  of  very  fine  subdivision  are 
efficacious,  not  the  coarser  subdivisions.  Wolfrom  especially  lauds  its 
beneficial  effects  in  pus  cocci  infection,  in  angina,  and  also  in  many 
cases  of  infection  at  the  joints. 

In  support  of  the  contention  that  it  is  the  very  fine  subdivisions 
that  are  efficacious  for  medicinal  purposes,  Henri  found  that  many 
bacilli  are  retarded  in  their  growth  by  finer  subdivisions  of  silver  sols 
even  at  a  dilution  of  1  to  50,000,  but  that  the  coarser  had  little  effect. 
The  effect  of  collargol  is  probably  to  be  attributed  to  the  formation 
of  silver  ion  at  the  great  dilution.  As  the  ion  particles  are  removed 
more  could  be  formed  from  the  submicrons  present  by  chemical 
influences,  such  as  oxidation,  etc.  The  colloidal  silver  would  therefore 
act  for  a  long  time  as  a  storehouse  for  the  ions.  The  ions  would 
not  be  present  in  large  enough  concentration  to  injure  the  tissues. 
However  the  cause  of  the  therapeutic  effect  may  lie  beyond  this. 
The  silver  may  excite  the  organism  to  form  antibodies,  or  cause  an 
increase  of  metathesis. 

Colloidal  Metals  According  to  the  Method  of  Paal 

Paal  and  Amberger,*  with  the  help  of  protective  colloids,  have 
succeeded  in  preparing  colloidal  solutions  of  the  platinum  group, 
Pt,  Os,  Pd,  Ir.  These  will  be  collectively  dealt  with.  The  same  pro- 
tective colloids,  viz.,  sodium  salts  of  protalbinnic  and  lysalbinnic  acid, 
were  employed,  as  in  the  case  of  colloidal  gold  and  silver.  Reversible 
colloids  of  great  stability  containing  as  much  as  50  to  70  per  cent  metal 
were  thus  obtained.  They  are  not  precipitated  by  even  10  per  cent 
sodium  chloride  solution  but  may  eventually  be  thrown  down  if  the 
concentration  of  the  electrolyte  is  increased  sufficiently.  Some  of 
these  solutions  are  not  at  all  precipitated  by  calcium  chloride.  Acids, 
on  the  other  hand,  throw  down  a  precipitate  soluble  in  water  or  in  dilute 
alkalis.  The  color  of  the  solutions  is  generally  dark  brown  or  black. 
Paal  and  his  collaborators  have  carried  out  some  very  interesting  re- 
actions with  these  metal  colloids  that  deserve  further  mention. 

Preparation.  —  The  preparation  of  colloidal  platinum  according  to 
the  method  of  Paal  will  be  discussed  as  an  example.  The  sodium  salt 
of  lysalbinnic  acid  is  dissolved  in  three  times  the  amount  of  water  and 
2  grams  of  hydrogen  platinic  chloride  dissolved  in  water  are  added. 
Enough  sodium  hydroxide  is  added  to  unite  with  the  chloride.  The 

*  C.  Paal  und  C.  Amberger:  Ber.,  37,  124-139  (1904);  Journ.  f.  prakt.  Chemie 
(2),  71,  358-365  (1904);  Ber.,  38,  1398-1405  (1905);  40,  1392-1404  (1907). 


124  CHEMISTRY  OF  COLLOIDS 

reddish  brown  liquid  so  obtained  is  next  treated  with  hydrazine  hydrate. 
Nitrogen  is  evolved  and  after  standing  for  about  5  hours  the  solution 
is  dialyzed.  It  is  then  evaporated  on  a  water  bath  and  eventually 
in  vacuum.  The  residue  is  a  black,  brittle,  lustrous  mass  soluble  in 
water. 

Paal  and  Ambronn  have  also  tried  other  mixtures  and  instead  of 
lysalbinnic  acid  have  employed  the  sodium  salt  of  protalbinnic  acid. 
Colloidal  palladium  was  easily  obtained  by  this  method.  The  evapo- 
ration took  place  at  60  to  70  degrees  and  the  dehydration  over  sulfuric 
acid  in  vacuum.  The  residue  consisted  of  black,  easily  soluble  lamellae. 
The  concentration  of  the  metal  can  be  achieved  by  precipitating  with 
acetic  acid,  redissolving  in  sodium  hydroxide,  and  subsequent  dialysis. 
Similarly  colloidal  osmium  and  iridium  may  be  prepared.  The  last- 
named  was  made  by  the  use  of  sodium  amalgam  as  the  reducing  agent. 
Osmium  colloid  cannot  be  obtained  free  from  oxide  unless  the  dried 
residue  is  further  reduced  by  the  use  of  hydrogen  at  30  to  40  degrees. 
The  dried  residue  gradually  loses  weight  owing  to  the  formation  of 
volatile  osmium  tetr oxide. 

Colloidal  Palladium.  —  Palladium  hydrosol  is  a  brown  liquid,  at 
times  almost  black,  and  contains  for  the  most  part  amicroscopic  par- 
ticles. A  small  portion  passes  through  a  collodion  filter  that  will  retain 
dilute  hemoglobin  solutions,  showing  that  the  particles  are  extremely 
small.  The  color  of  the  filtrate  is  almost  the  same  as  dilute  solutions 
of  the  original  liquid,  although  sometimes  the  color  is  a  trifle  more 
reddish.  Dilute  solutions  at  a  concentration  of  0.0005  per  cent  palladium 
are  still  colored  in  layers  more  than  1  cm.  thick,  and  have  a  pronounced 
catalytic  action. 

Colloidal  Palladium  Containing  Hydrogen.  —  A  palladium  hydro- 
gen complex  may  be  prepared  from  ordinary  palladium,  palladium 
black,  and  also  colloidal  palladium  by  both  wet  and  dry  methods.  In 
a  dry  way  *  it  is  made  by  conducting  hydrogen  over  solid  colloidal 
palladium  at  60  to  110  degrees  whereby  three  atoms  of  palladium  take 
up  about  one  atom  of  hydrogen.  By  heating  to  130  or  140  degrees  the 
hydrogen  may  be  driven  off  with  a  current  of  carbon  dioxide  without 
damaging  the  solubility  of  the  residue.  It  is  a  well-known  fact  that 
the  palladium  hydrogen  complex  is  a  good  reducing  agent.  In  a 
very  fine  state  of  subdivision  it  is  pyrophoric.  The  hydrosol  has 
been  ma.de  by  Paal  and  Gerum  |  by  treating  the  pure  palladium 
hydrosol  with  hydrogen.  It  lias  been  found  that  colloidal  palladium 
will  take  up  from  926  to  2952  volumes  of  hydrogen,  while  according 

*  C.  Paal  und  C.  Amberger:  Ber.,  38,  1399  (1905). 
t  C.  Paal  und  J.  Gerum:  Ber.,  41,  805-817  (1908). 


INORGANIC  COLLOIDS  125 

to  Mond,  Ramsay,  and  Shields  *  palladium  black  adsorbs  only  873 
volumes.  The  amount  of  hydrogen  obtained  by  heating  is  less  than 
that  adsorbed. 

Catalytic  Properties  of  the  Platinum  Group  in  a  Colloidal  State 

Very  interesting  are  the  investigations  of  Paal  in  collaboration  with 
Amberger,f  Gerum,t  and  Roth  §  on  the  catalytic  effect  of  the  hydro- 
sols  of  this  group,  especially  the  activation  by  hydrogen.  The  effect 
of  the  different  hydrosols  on  the  decomposition  of  hydrogen  peroxide 
was  determined  and  the  results  indicate  that  the  series  is 

Os  >  Pt  >  Pd  >  Ir. 

A  previous  treatment  with  hydrogen  may  increase  the  effect  very 
greatly.  This  was  first  discovered  in  the  reduction  of  nitrobenzol 
whereby  a  considerable  quantity  of  aniline  was  formed  by  the  palladium 
hydrogen  complex.  This  property  is  not  possessed  by  palladium  black 
nor  palladium  foil.  In  order  to  compare  the  effect  of  the  different 
hydrosols  the  amount  of  hydrogen  used  for  reduction  in  the  unit  of 
time  was  calculated.  From  this  could  be  calculated  the  volume  re- 
lations between  the  activating  hydrogen  and  the  palladium.  By 
"  Aktivierungzahl "  the  authors  understand  the  amount  of  hydrogen  in 
cubic  centimeters  used  by  one  cubic  centimeter  of  Pd  in  one  hour.  This 
number  varied  from  12,000  to  32,000  and  increased  with  the  temper- 
ature. It  also  depended  very  largely  upon  the  state  of  the  colloid;  old 
preparations  seemed  to  work  better  than  freshly  prepared  samples. 
It  is  remarkable  that  aniline  can  be  made  by  this  method,  because 
Bredig  found  that  it  is  a  poison  for  catalyzers.  The  explanation  is  that 
aniline  is  at  first  a  poison  but  in  larger  quantities  has  an  accelerating 
effect. 

The  experimental  conditions  were  that  10  cc.  palladium  sol,  2  gms. 
nitrobenzol,  and  10  cc.  alcohol  were  put  into  a  small  flask  with  a  con- 
denser and  hydrogen  conducted  in  continuously  at  70  degrees.  In  a 
similar  manner  the  activation  of  platinum  was  found  to  be  6700  to 
37,000,  that  of  iridium  2000  to  4000,  silver  and  osmium  very  small, 
while  gold  and  copper  are  not  affected  by  hydrogen. 

The  Reduction  of  Unsaturated  Organic  Compounds.  —  The  addition 
of  hydrogen  to  unsaturated  compounds  with  the  aid  of  the  platinum 

*  L.  Mond,  W.  Ramsay  und  J.  Shields:  Zeit.  f.  anorg.  Chemie,  16,  325-328  (1898). 
See  also  Zeit.  f.  phys.  Chemie,  26,  109-112  (1898). 

t  C.  Paal  und  C.  Amberger:  Ber.,  38,  1406-1409,  2414  (1905);  40,  2201-2208 
(1907). 

t  C.  Paal  und  J.  Gerum:  Ber.,  40,  2209-2220  (1907);  41,  2273-2282  (1908). 

§  C.  Paal  und  K.  Roth:  Ber.,  41,  2283-2291  (1908). 


126 


CHEMISTRY  OF  COLLOIDS 


metals  has  been  known  for  a  long  time.  Up  to  the  time  of  Paal,  how- 
ever, the  metals  had  not  been  used  for  this  purpose  in  colloidal  form, 
but  usually  as  platinum  black.  The  reduction  with  the  help  of  Co  and 
Ni  as  catalytic  agents  according  to  Sebatier  and  Senderens,  also  Le- 
prince  and  Siveke,*  does  not  belong  here.  Debus  f  and  Fokin,|  and  also 
Willstatter,  §  recognized  the  principles  of  the  reaction,  and  used  it  in 
the  determination  of  constitution.  Paal  showed  that  other  reductions 
of  unsaturated  compounds,  such  as  acids,  aldehydes,  ketones,  diketones, 
nitriles,  etc.,  could  be  carried  out  even  more  satisfactorily  than  that  of 
nitrobenzol. 

The  distinct  advantage  of  colloidal  metals  over  the  powder  form  is 
doubtless  due  to  the  increase  of  surface  and  to  the  fact  that  the  particles 
of  metal  and  the  molecules  to  be  reduced  are  in  more  intimate  contact. 
Both  of  these  circumstances  should  aid  the  catalytic  action. 

The  reduction  of  maleic  and  fumaric  acids  will  be  discussed  as  ex- 
amples. The  pure  acid  in  alcoholic  solution  is  not  reduced  by  hydrogen. 

On  the  other  hand  the  reaction  is  al- 
most complete  if  the  acids  are  neu- 
tralized with  sodium  carbonate  and 
hydrogen  introduced  in  the  presence 
of  colloidal  palladium.  The  progress 
may  be  followed  easily  in  a  glass  buret 
that  is  attached  to  an  apparatus  for 
shaking  the  reacting  mixture,  such  as 
may  be  seen  in  Fig.  20.  The  mixture 
is  brought  into  the  vessel  A,  which  is 
kept  in  motion  by  a  shaking  apparatus. 
C  is  a  capillary  tube  and  B  is  a  gas 
buret.  The  air  is  first  driven  off  by 
hydrogen  and  the  entire  apparatus 
FlG\  f '  Apparatus  for  Reduction  gaturated  with  it  by  means  of  shaking. 
with.  Colloids  according  to  Paal.  T>  «  T  ,  i  j_i  i  A 

Palladium  must  be  in  the  vessel  A 

while  the  shaking  is  going  on.  The  substance  to  be  reduced  is  now 
brought  into  the  vessel  A,  care  being  taken  not  to  introduce  any  air. 
The  shaking  is  continued  until  there  is  no  longer  a  rise  of  liquid  in  B. 
The  difference  in  the  two  readings  on  the  buret  gives  the  number  of 

*  Leprince  u.  Siveke:   D.  R.  P.  141029  (1902);  C.  (1903),  1,  1199. 

t  H.  Debus:  Liebigs  Annalen,  128,  200  ff.(1863). 

j  S.  Fokin:  C.,  11,  758  (1906);  II,  1324  (1907).  Journ.  russ.  phys.-chem.  Ges., 
38,  419  ff.;  39,  607  ff. 

§  R.  Willstatter  und  E.  W.  Mayer:  Ber.,  41,  1475,  2199  (1908).  Derselbe  und 
E.  Hauenstein:  Ibid.,  42,  1850  (1909).  Derselbe  und  E.  Waser:  IUd.,  43,  1176 
(1910);  44,  3423  (1911). 


INORGANIC  COLLOIDS  127 

cubic  centimeters  used  in  the  reduction.  Vessel  A  has  a  capacity  of 
about  150  cc.  The  following  proportions  are  suitable  for  the  experi- 
ment: 0.1  g.  fumaric  acid  dissolved  in  10  cc.  water,  neutralized  with 
sodium  carbonate,  and  0.08  g.  of  solid  palladium  colloid  dissolved  in  10 
cc.  water.  Many  organic  substances  may  be  reduced  by  this  means. 

Colloidal  Copper 

Colloidal  copper  is  interesting  because  of  its  relation  to  copper  ruby 
glass.*  The  constitution  of  this  glass  has  been  long  under  dispute. 
Some  authors  have  held  that  the  copper  is  in  the  form  of  cuprous  oxide, 
while  others  believe  the  free  metal  is  present.  The  deciding  of  this 
point  is  scarcely  possible  by  analysis,  but  light  has  been  thrown  on  it 
since  colloidal  copper  has  been  made  having  the  same  color  as  copper 
ruby  glass.  Lottermoserf  and  Billitzert  obtained  brown,  Gutbier§ 
blue  hydrosols  by  the  reduction  of  copper  salts  or  by  electrical  colloida- 
tion.  Paal  and  Leuze  If  have  prepared  both  red  and  blue  hydrosols  by 
the  reduction  of  colloidal  copper  oxide  with  hydrogen  or  hydrazine 
hydrate.  The  necessary  copper  oxide  was  made  from  copper  sulfate 
to  which  potassium  hydroxide  and  the  sodium  salt  of  lys-  or  protalbin- 
nic  acid  had  been  added.  The  resulting  dark  blue  liquid  was  dialyzed 
and  evaporated.  The  reduction  of  copper  oxide  in  a  wet  way  by 
means  of  hydrazine  takes  place  in  two  stages.  At  first  an  orange 
colored  milky  liquid  is  obtained  that  doubtless  contains  cuprous  oxide. 
On  further  reduction  the  liquid  becomes  clear  and  is  colored  deep  red. 
Paal  and  Leuze  obtained  liquids  in  this  manner  that  were  quite  black 
in  reflected  but  deep  red  in  transmitted  light.  This  agrees  perfectly 
with  thejsolor  of  copper  ruby  glass.  The  author  was  able  to  convince 
himself  with  an  apparatus  of  his  own  make  that  the  red  liquid  gives 
the  absorption  lines  near  line  D  that  are  characteristic  of  copper  ruby 
glass.  The  absorption  lines  were  somewhat  wider  and  the  maximum 
displaced  slightly  toward  line  C,  which  would  indicate  that  a  partial 
coagulation  had  set  in  whereby  flocculent  complexes  were  formed, 
similar  to  the  case  of  gold  gelatin  solutions  on  evaporation.  Copper 
hydrosols  should  be  investigated  further  with  the  ultramicroscope  and 
also  from  the  standpoint  of  spectral  analysis. 

Red  copper  hydrosol  behaves  differently,  depending  upon  whether  it 
is  made  with  the  sodium  salt  of  prot-  or  lysalbinnic  acid.  The  first 

*  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  Chapter  XVI  (1905). 

t  A.  Lottermoser:  Journ.  f.  prakt.  Chemie  (2),  59,  489-493  (1899). 

J  J.  Billitzer:  Ber.,  35,  1929-1935  (1902). 

§  A.  Gutbier:  Zeit.  f.  anorg.  Chemie,  32,  355  (1902). 

II  C.  Paal  und  W.  Leuze:  Ber.,  39,  1545-1549,  1550-1557  (1906). 


128  CHEMISTRY   OF  COLLOIDS 

gives  an  olive  green  color  with  sodium  chloride,  a  phenomenon  that 
has  been  observed  in  the  case  of  copper  ruby  glass.  The  second  be- 
comes blue  when  treated  in  a  similar  manner.  To  sum  up,  the  color 
of  copper  hydrosols  may  be  red,  brown,  green,  or  blue. 

Other  Metal  Colloids 

Among  other  colloidal  metals  tungsten  deserves  mention  because  of 
its  importance  in  electric  light  bulbs.  A  method  for  making  the 
material  for  these  filaments  has  been  worked  out  by  Kuzel.*  The 
colloidal  metal  is  obtained  by  continued  grinding  and  by  alternate 
treatments  with  acid  and  alkali.  By  this  process  the  particles  become 
so  small  that  the  metal  finally  forms  a  hydrosol.  On  precipitation  of 
the  sol  a  plastic  mass  is  obtained  that  is  next  pressed  out  through  fine 
holes,  by  which  process  threads  are  formed.  The  lamps  are  economical 
and  last  longer  than  the  old  varieties.  Molybdenum,  silicon,  titanium, 
and  thorium  may  be  prepared  in  a  colloidal  state  by  the  same  method. 

Wedekind  f  has  demonstrated  that  zirconium  sol  may  be  made  by 
etching  the  metal  with  hydrochloric  acid.  A  powder  is  thus  obtained 
that  goes  into  colloidal  solution  on  washing  with  water.  This  hydrosol 
gives  very  peculiar  reactions  with  electrolytes.  Most  acids  precipitate 
it,  but  tartaric  and  picric  acids  will  not.  The  hydroxides  of  the  alkali 
metals  precipitate  it  immediately,  but  ammonia  water  does  so  very 
slowly.  Most  neutral  electrolytes  have  little  or  no  effect.  It  is  prob- 
able that  the  treatment  with  HC1  forms  a  protective  colloid  which 
causes  the  unusual  behavior. 

*  A.  Lottermoser:  Chem.-Ztg.,  311~(1908).     Koll.-Zeit.,  2,  347  (1908). 
t  E.  Wedekind:  Koll.-Zeit.,  2,  289-293  (1908). 


CHAPTER  VI 
COLLOIDAL  NONMETALS 

Colloidal  Sulfur 

SOBRERO  and  Selmi,*  Wackenroder,f  and  Debus  t  have  studied  the 
reactions  between  hydrogen  sulfide  and  sulfurous  acid,  and  have  found 
that  in  addition  to  thionic  acids  colloidal  sulfur  is  formed.  Such 
hydrosols  are  generally  turbid,  and  contain  a  portion  of  the  sulfur  in 
microscopic  form,§  although  most  of  it  remains  dissolved  probably  as 
amicrons.  Raffo  Tf  has  recently  devised  a  method  for  making  perfectly 
clear  colloidal  sulfur  solutions  that  have  a  considerable  stability  toward 
electrolytes.  The  procedure  is  as  follows: 

50  g.  of  crystallized  sodium  thiosulfate  in  30  cc.  of  water  are  added 
drop  by  drop  to  70  g.  1.84  sulfuric  acid.  The  mixture  is  cooled  and 
30  cc.  of  water  are  added.  The  mixture  is  then  warmed  for  10  min- 
utes at  80  degrees  and  filtered  through  glass  wool.  The  precipitate  is 
cooled,  washed  with  water,  warmed,  and  again  filtered.  This  process 
is  repeated  a  number  of  times.  The  purified  sulfur  mass  is  next  cen- 
trifugalized  and  finally  dissolved  in  water.  The  solution  is  neutralized 
with  sodium  carbonate,  whereby  most  of  the  sulfur  falls  out.  The  liquid 
contains  about  1  per  cent  sulfur  and  6  per  cent  sodium  sulfate.  The 
precipitated  sulfur  dissolves  completely  in  water,  forming  a  clear  liquid 
that  contains  4.5  per  cent  sulfur  and  1.5  per  cent  sodium  sulfate.  By 
dialysis  the  salt  may  be  partially  removed;  however,  continued  puri- 
fication will  precipitate  out  the  entire  sulfur  content. 

The  liquid  mentioned  above  containing  1  per  cent  sulfur  and  6  per 
cent  sodium  sulfate  will  give  a  precipitate  on  the  addition  of  a  half- 
normal  solution  of  the  potassium  salt  of  sulphuric,  nitric,  or  hydro- 
chloric acid.  The  corresponding  ammonium  salts  do  not  have  this 
property.  We  have  here  to  do  with  a  very  specific  reaction  toward 
electrolytes.  While  the  colloidal  solutions  already  discussed  are  pre- 

*  A.  Sobrero  et  F.  Selmi:  Annales  de  Chim.  et  de  Phys.  (3),  28,  210-214  (1850). 
t  Wackenroder:  Archiv  d.  Pharmazie,  48,  140,  272  (1846);  Annalen  d.  Chemie  u. 
Pharmazie,  60,  189  (1846). 

t  H.  Debus:  Liebigs  Annalen,  244,  76-189  (1888). 

§  J.  Stingl  und  Th.  Morawski:  Journ.  f.  prakt.  Chemie  (2),  20,  76-105  (1879). 
If  M.  Raffo:  KoU.-Zeit.,  2,  358-360  (1908). 

129 


130 


CHEMISTRY  OF  COLLOIDS 


cipitated  by  small  amounts  of  electrolytes,  these  same  small  amounts 
are  necessary  to  keep  colloidal  sulfur  in  solution.  Larger  concentra- 
tions on  the  other  hand  precipitate  the  sulfur.  Sulfur  solutions  there- 
fore resemble  those  of  globulin,  which  are  also  not  soluble  in  pure 
water,  but  require  a  small  amount  of  electrolyte.  Moreover,  sulfur 
solutions  resemble  those  of  glue  and  soluble  starch  in  that  the  sulfur 


30 


Solubility  of  sulfur. 


is  more  soluble  in  hot  than  in  cold  solution.  On  the  contrary 
sulfur  differs  from  glue  and  soluble  starch  in  its  behavior  toward 
larger  concentrations  of  electrolytes,  as  shown  by  the  researches  of 
Svedberg.* 

The  author  has  observed  the  peculiar  behavior  of  sulphur  hydrosols 
toward  collodion  membranes.     The  latter  remove  almost  all  of  the 
sulfur  from  the  liquid.     Colloidal  sulfur  had  a  pressure  against  its 
*  The  Svedberg:  Koll.-Zeit.,  2,  49-54  (1909). 


COLLOIDAL  NONMETALS  131 

filtrate  represented  by  136  mm.  of  water  but  sank  toward  the  end  of 
a  month  to  100  mm.  At  the  end  the  inner  liquid  contained  mostly 
amicrons. 

As  shown  by  Raffo  sulfur  comes  out  of  colloidal  solution  and  forms, 
on  long  standing,  well-defined  crystals  having  a  normal  melting  point. 
These  crystals  dissolve  easily  in  carbon  disulfide  but  do  not  in  water. 
In  an  endeavor  to  purify  colloidal  sulfur  the  author  noticed  that  it 
lost  its  solubility.  Just  as  long  as  it  retains  this  solubility  in  water 
the  colloid  appears  to  be  amorphous  and  somewhat  plastic;  it  is  also 
negatively  charged.  From  this  one  must  assume  that  the  amicrons 
hold  tenaciously  to  the  impurities,  water  and  electrolytes,  and  that 
these  substances  are  responsible  for  the  solubility  in  water.  Svedberg 
has  shown  that  precipitation  with  sodium  chloride  and  decanta- 
tion  completely  substitutes  the  chloride  ion  for  the  sulfate  ion,  but 
that  the  colloid  still  retains  its  solubility  in  water.  The  dependence 
of  the  solubility  on  the  temperature  and  the  concentration  of  the 
electrolyte  is  shown  by  the  curves  in  Fig.  21.  It  is  also  quite 
remarkable  that  the  colloidal  sulfur  obtained  by  Sobrero  and  Selmi 
was  reversibly  precipitated  by  sodium  salts  but  irreversibly  by  the 
salts  of  potassium. 

The  presence  of  colloidal  sulfur  in  the  same  solution  with  suspended 
sulfur  was  shown  by  Debus  on  evaporating  the  hydrosol.  When  about 
two-thirds  of  the  water  is  evaporated  the  solution  is  clear  and  yellow. 
Dialysis  separates  out  sulfur  just  as  in  the  case  of  the  preparation  of 
Raffo.  An  elaborate  investigation  of  the  fractional  precipitation  of 
sulfur  by  Sven  Oden  *  has  shown  that  the  larger  particles  are  more 
easily  precipitated  than  the  finer  ultramicrons. 

Colloidal  Selenium 

In  1885,  H.  Schulze  f  prepared  hydrosols  of  selenium  by  treating  selen- 
ium dioxide  with  sulfurous  acid.  If  sufficiently  concentrated  solutions 
are  employed  a  precipitate  is  formed  that  will  again  dissolve  in  water. 
The  hydrosols  are  fairly  stable  if  kept  in  the  dark,  are  clear,  and  appear 
red  in  transmitted  light.  On  long  standing  the  liquid  becomes  turbid 
and  separates  into  two  layers  with  a  sharply  defined  boundary.  The 
lower  one  contains  most  of  the  selenium  and  has  a  much  higher  specific 
gravity.  By  tipping  the  vessel  the  lower  layer  flows  toward  the  lowest 
part  of  the  bottom  much  as  water  does  under  an  oil  layer.  However, 
it  is  still  miscible  with  the  upper  layers  and  contains  selenium  particles 
that  have  not  yet  united  but  are  separated  from  each  other  by  water. 

*  Sven  Ode"n:  Zeit.  f.  phys.  Chemie,  78,  682-707  (1912). 

t  H.  Schulze:  Journ.  f.  prakt.  Chemie  (2),  32,  390-407  (1885). 


132  CHEMISTRY  OF   COLLOIDS 

Colloidal  selenium  is  sensitive  to  light.  If  a  vessel  is  partially  cov- 
ered with  black  paper  and  allowed  to  stand  in  the  light  for  a  long  time 
a  lustrous  precipitate  of  selenium  is  seen  in  the  illuminated  part  of  the 
liquid.  Electrolytes  precipitate  colloidal  selenium,  while  boiling  causes 
the  color  to  change  to  blue.  When  this  blue  liquid  is  shaken  with 
carbon  disulfide  selenium  separates  out  in  the  layer  between  the  water 
and  the  carbon  disulfide  but  does  not  appear  to  be  dissolved.  If  the 
red  liquid  is  shaken  with  carbon  disulfide  the  selenium  separates  in  the 
dividing  layer  but  finally  dissolves  in  the  carbon  disulfide.  Especially 
beautiful  red  preparations  that  are  rather  stable  toward  electrolytes 
are  obtained  by  the  method  of  Paal.* 

*  C.  Paal  und  C.  Koch:  Ber.,  38,  526-534  (1905). 


CHAPTER  VII 
COLLOIDAL   OXIDES 

SCAKCELY  any  other  group  manifests  such  varying  properties  and 
different  reactions  as  do  the  colloidal  oxides.  They  exist  in  all  possible 
states  of  subdivision  from  the  ionic  to  coarse  suspensions.  The  hydro- 
sols  may  be  reversible  or  irreversible,  stable  or  unstable,  sensitive  to 
electrolytes  or  not,  and  finally  some  are  charged  positively  and  some 
negatively.  The  properties  of  these  colloids  are  to  a  much  greater 
extent  dependent  upon  the  nature  of  the  substance  than  are  those  of 
the  metals,  sulfides,  or  salts.  In  contradistinction  to  these  properties 
that  depend  upon  the  substance  is  the  relation  between  properties 
subject  to  gradual  change  because  of  concentration,  previous  history, 
or  amount  of  peptising  material  present.  This  latter  group  of  proper- 
ties may  vary  over  a  wide  range,  and  this  makes  the  description  of  these 
colloids  all  the  more  difficult.  We  are  met  at  the  outset  by  an  enor- 
mous number  of  facts,  a  comprehensive  presentation  of  which  would 
serve  to  confuse  rather  than  to  elucidate.  This  will  be  possible  only 
after  the  systematic  discussion  of  individual  members. 

In  order  to  give  an  approximate  idea  of  the  varying  relations  we  are 
confronted  with  here,  the  author  has  confined  himself  to  a  description 
of  Graham's  colloidal  oxides,  without  making  any  attempt  to  be  compre- 
hensive. Such  colloidal  oxides  have  been  chosen  as  are  of  general 
importance.  For  instance  under  colloidal  silicic  acid  the  formation  of 
the  most  finely  divided  gels  is  discussed;  under  the  hydrogel  of  iron 
oxide  the  investigations  of  Duclaux  on  conductivity  and  osmotic  pres- 
sure are  presented  and  remarked  upon.  Moreover  in  connection  with 
this  same  colloid  the  important  magneto-optical  and  ultramicroscopical 
researches  of  Cotton  and  Mouton  on  the  construction  of  the  ultramicron 
are  taken  up.  Under  colloidal  stannic  acid  the  relation  of  a.  to  /3  forms, 
under  the  purple  of  Cassius  the  similarity  of  colloidal  unions  to  chemi- 
cal combinations,  and  under  zirconium  oxide  the  influence  of  the  transi- 
tion of  dissolved  crystalloidal  substances  to  the  colloidal  state  on  the 
reactions  and  properties,  are  discussed. 

The  tendency  of  many  colloidal  oxides  to  appear  in  two  modifications 
has  been  already  referred  to.  We  are  not  yet  able  to  decide  definitely 
whether  we  have  here  to  deal  with  different  substances  in  the  chemical 

133 


134  CHEMISTRY  OF  COLLOIDS 

sense,  such  as  anhydrides,  hydrates,  polymers,  etc*.,  or  whether  it  is  a 
question  of  colloidal  chemical  properties  resulting  from  differences  in 
degree  of  dispersion.  Many  facts,  however,  point  to  the  correctness 
of  the  latter  view  as  will  be  shown  in  this  and  following  chapters. 

A.   Colloidal  Silicic  Acid 

The  hydrogel  of  silicic  acid  appears  in  nature  in  the  form  of  opal, 
hydrophane,  and  tabashir.  The  hydrogel,  as  is  well  known,  is  precipi- 
tated in  a  jelly  form  when  most  silicates  are  decomposed.  By  com- 
pletely dehydrating  the  residue  an  insoluble  powder  remains. 

Preparation  and  Properties  of  the  Hydrosol.  —  The  hydrogel  may 
be  prepared  either  by  Graham's  *  method  of  dialyzing  a  mixture  of 
water  glass  and  hydrochloric  acid,  according  to  Grimaux  f  by  the  decom- 
position of  the  methyl  ester  of  silicic  acid  with  water,  or  by  the  effect 
of  water  on  silicon  chlorides  or  silicon  sulfide.J  Graham's  method  is  as 
follows:  10  per  cent  sodium  metasilicate  and  10  per  cent  HC1  are  mixed 
together  under  constant  shaking.  In  order  to  determine  the  correct 
relations  for  mixing,  a  trial  experiment  must  be  performed.  Water 
glass  is  run  into  a  definite  amount  of  the  acid  until  the  solution  begins 
to  solidify,  the  shaking  being  continuous  while  the  addition  is  being 
made.  About  two-thirds  the  amount  of  water  glass  necessary  to  cause 
solidification  should  be  used  to  prepare  the  hydrogel.  If  it  is  found 
during  dialysis  that  a  jelly  forms,  the  water  glass  employed  should  be 
diluted  or  the  HC1  used  in  more  concentrated  form.  The  hydrosol 
may  be  purified  to  a  large  extent  by  dialysis,  and  further  concentrated 
if  desired  by  evaporation. 

A  well  purified  and  not  too  dilute  hydrosol  of  silicic  acid  may  be 
looked  upon  as  an  unstable  system  that  has  a  constant  tendency  towards 
coagulation,  and  this  coagulating  tendency  is  more  pronounced  in  the 
more  concentrated  solutions.  It  is  seldom  possible  to  increase  the 
concentration  to  over  10  per  cent.  Graham  once  prepared  a  solution 
having  14  per  cent  colloidal  silicic  acid  but  it  was  so  unstable  that  in- 
different substances  such  as  graphite  or  carbon  dioxide  served  to  coagu- 
late it.  Solutions  under  1  per  cent  may  be  kept  for  years,  and  are  fairly 
stable  toward  electrolytes.  Some  of  the  latter  cause  immediate  precipi- 
tation, while  others  do  so  only  after  several  hours. 

Well-prepared  silicic  acid  hydrosols  are  quite  clear  colorless  liquids 
that  scarcely  exhibit  any  inhomogeneity  under  the  ultramicroscope. 
If  they  are  dialyzed  with  parchment  membranes  they  frequently  become 

*  Th.  Graham:  Liebigs  Annalen,  121,  36  (1862). 
t  E.  Grimaux:   Compt.  rend.,  98,  1434-1437  (1884). 
J  E.  Ebler  und  M.  Fellner:  Ber.,  44,  1915-1918  (1911). 


COLLOIDAL  OXIDES  135 

turbid  and  contain  submicrons.  Jordis  *  considers  it  impossible  to  pre- 
pare colloidal  solutions  of  silicic  acid  pure  where  the  concentration  is 
over  2  per  cent.  He  attributes  the  stability  of  Graham's  solutions  to 
the  presence  of  foreign  substances,  and  even  goes  so  far  as  to  doubt  the 
existence  of  Si02  as  such  in  the  solution.  He  thinks  the  alkali  or  acid 
forms  compounds  with  the  silicon  dioxide.  This  assumption  seems 
unnecessary  to  explain  the  stability  of  pure  hydrosols.  The  relation 
may  be  similar  to  the  case  of  supersaturated  solutions  where  the  sta- 
bility depends  upon  the  degree  of  supersaturation.  Uncontrollable 
factors  enter  into  the  crystallization  of  supersaturated  solutions,  such 
as  dust  particles,  and  it  is  quite  possible  that  conditions  of  one  experi- 
menter cannot  be  duplicated  by  another  when  working  with  hydrosols. 

With  regard  to  the  purification  of  silicic  acid  hydrosols  by  dialysis, 
Zsigmondy  arid  Heyer  f  have  found  that  the  chloride  ion  may  be  com- 
pletely removed  but  that  it  is  much  more  difficult  to  get  the  colloid 
free  from  sulfate.  After  long  dialysis  there  remained  2  to  3  mols.  of 
sodium  sulfate  for  every  mol.  Si02.  Such  traces  of  electrolyte  are  almost 
impossible  to  get  rid  of.  The  hydrosol  containing  this  quantity  of 
sulfate  could  be  concentrated  by  evaporation  until  the  silicic  acid  is 
from  6  to  12  per  cent  of  the  entire  mass.  Well  purified  hydrosol  de- 
presses the  freezing  point  very  little,  so  that  Sabanejeff  J  obtained  a 
molecular  weight  of  49,000  by  this  method.  Bruni  and  Pappada  § 
could  not  observe  any  lowering  of  the  freezing  point  with  their  prep- 
arations. The  more  sensitive  direct  measurement  of  the  osmotic 
pressure  shows  that  the  colloid  exhibits  such  against  its  filtrate.  The 
pressure  becomes  smaller  as  coagulation  proceeds. 

Electric  Charge.  —  The  amicrons  of  the  hydrosol  are  charged  nega- 
tively and  migrate  toward  the  anode  in  neutral,  alkaline,  or  weakly 
acid  solution.  During,  the  electrolysis  of  neutral  solutions  silicic  acid 
does  not  separate  out  at  the  anode  as  stannic  acid  and  many  other 
colloids  do,  but  falls  slowly  to  the  bottom  in  striations  much  after  the 
nature  of  sulfuric  acid. 

Precipitation  with  Electrolytes.  —  As  already  pointed  out  silicic 
acid  is  not  immediately  precipitated  by  all  electrolytes.  Hydrochloric 
acid,  chlorides  of  the  alkali,  and  alkaline  earth  metals  give  no  precipi- 
tate but  may  cause  gelatinization  after  standing  for  hours,  days,  or 
perhaps  weeks.  It  is  precipitated  immediately  by  barium  hydroxide, 

*  E.  Jordis:  Zeit.  f.  anorg.  Chemie,  35,  16-22  (1903). 

t  R.  Zsigmondy  und  R.  Heyer:  Zeit.  f.  anorg.  Chemie,  68,  169-187  (1910). 

J  A.  Sabanejeff:  Journ.  d.  russ.  phys.-chem.  Ges.,  21,  515-525  (1889);  Ber.,  23, 
R.,  87  (1890). 

§  G.  Bruni  e  N.  Pappada:  Atti  della  R.  Accad.  dei  Lincei  Roma  (5),  9,  354^358 
(1900);  Gazzetta  chimica  ital.,  31,  1,  244-252  (1901). 


136  CHEMISTRY  OF   COLLOIDS 

concentrated  solutions  of  ammonium  sulfate,  dilute  solutions  of  egg 
albumin,  glue,  and  many  basic  dyestuffs  such  as  Methylene  Blue. 
Regarding  the  effect  of  the  salts  of  the  alkali  metals  on  the  precipita- 
tion, Pappada  *  found  that  the  anions  played  little  part  but  that  the 
influence  of  the  cathions  increased  with  the  molecular  weight,  caesium 
having  the  greatest  and  lithium  the  least. 

Protective  Influence  of  Silicic  Acid.  —  Silicic  acid  hydrosol  does  not 
act  as  a  protective  colloid  for  colloidal  gold.  It  does  not  hinder  the 
change  of  color  by  sodium  chloride,  neither  does  it  prevent  the  forma- 
tion of  a  turbidity  nor  a  precipitate  in  silver  chloride  solutions,  unless 
the  amount  of  chloride  ion  is  extremely  small.  In  the  latter  case  the 
reaction  is  obscure,  and  is  not  protective  action  in  the  ordinary  sense. 
This  is  shown  clearly  by  the  addition  of  small  quantities  of  suitable 
electrolytes  such  as  sulphuric  acid  or  potassium  sulfate,  when  a  turbid- 
ity is  formed  and  precipitation  occurs.  The  presence  of  two  drops  of  con- 
centrated sulfuric  acid  in  10  cc.  of  the  hydrosol  enables  mere  traces  of 
chloride  to  be  detected  with  silver  nitrate  almost  as  well  as  in  pure  water. 
Silicic  acid  hydrosols  also  prevent  the  formation  of  large  particles  of 
the  noble  metals.  Indeed  clear  metal  hydrosols  may  be  obtained  by 
this  method.  Real  protective  action  occurs  for  the  first  time  at  the 
moment  that  the  silicic  acid  is  being  precipitated.  Under  these  con- 
ditions the  change  of  color  of  gold  solutions  may  be  prevented  and 
therefore  the  formation  of  large  particles.f 

Transitions  of  Silicic  Acid.  —  Mylius  and  Groschuff  *  have  demon- 
strated that  silicic  acid  at  the  moment  of  its  formation  from  water 
glass  is  in  the  crystalloidal  state  and  gradually  goes  into  the  colloidal 
state  later.  The  newly  formed  silicic  acid  causes  an  appreciable  lower- 
ing of  the  freezing  point  that  gradually  decreases  as  the  formation  of 
the  colloid  proceeds.  The  transition  may  be  followed  by  means  of 
reagents.  For  instance  egg  albumin  causes  no  precipitation  in  freshly 
prepared  solutions,  but  does  so  after  the  same  solutions  have  stood  for 
some  time.  In  close  connection  with  this  is  the  loss  of  silicic  acid  dur- 
ing dialysis.  Zsigmondy  and  Heyer  §  found  that  in  the  Stern  dialyzer 
more  than  90  per  cent  of  the  silicic  acid  went  through  thin  collodion 
membranes  which  were  sufficiently  thick  to  prevent  completely  the 
passage  of  colloidal  silver. 

Crystallization  of  Silicic  Acid.  —  Several  authors  have  succeeded  in 
obtaining  crystals  at  higher  temperatures  from  both  the  gel  and  the 

*  N.  Pappad^,:  Gazzetta  chimica  ital.,  33,  272-276  (1903);  35,  78-86  (1905). 

t  F.  Kttspert:  Ber.,  35,  2815  (1902). 

}  F.  Myliue  und  E.  Groschuff:  Ber.,  39,  116-125  (1906). 

§  Zsigmondy  und  Heyer:  1.  c.  page  135. 


COLLOIDAL  OXIDES  137 

hydrogel  of  silicic  acid.  They  were  mostly  quartz  and  tridymite. 
Senarmont  *  obtained  quartz  crystals  by  heating  gelatinous  silicic  acid 
containing  traces  of  hydrochloric  acid  for  several  days  at  350  degrees. 
Chrustschoff  f  obtained  quartz  and  tridymite  by  heating  a  dialyzed 
solution  of  silicic  acid  at  250  degrees  for  six  months.  Bruhns  J  obtained 
quartz  and  tridymite  by  heating  for  10  hours  at  300  degrees  after 
ammonium  fluoride  had  been  added  to  the  dialyzed  hydrosol.  Although 
these  crystals  have  been  made  at  higher  temperatures  it  is  scarcely  to 
be  doubted  that  they  may  be  formed,  if  very  slowly,  at  the  temperature 
of  the  room.  Indeed  a  number  of  changes  that  occur  when  the  hydro- 
gel  is  kept  for  a  long  time  in  an  atmosphere  saturated  with  water  vapor, 
may  be  best  explained  on  the  assumption  of  the  formation  of  ultrami- 
croscopic  crystals  from  the  ultramicrons. 

Silicic  Acid  Gel 

The  gel  of  silicic  acid  originates  from  the  hydrosol  by  coagulation 
either  spontaneously  or  with  the  aid  of  electrolytes.  Its  properties  are 
quite  different  from  those  of  the  precipitates  of  metal  hydrosols.  The 
latter  are  powders,  and  the  particles  are  separated  from  the  water.  On 
the  other  hand  the  silicic  acid  hydrogel  always  contains  water  that 
cannot  be  completely  removed  by  filtering  or  pressure.  As  is  well  known 
the  water  may  be  driven  off  by  heating  but  the  gel  suffers  an  irreversi- 
ble change.  The  same  thing  holds  for  the  gel  of  other  oxides.  The 
water  is  more  firmly  held  than  by  the  gels  of  the  colloidal  metals. 

This  has  been  explained  on  the  grounds  that  we  are  dealing  with 
hydrates  and  not  with  oxides.  For  the  general  behavior  of  the  gels  it 
seems  indifferent  whether  the  nucleus  is  a  hydrate  or  not.  In  a  freshly 
prepared  gel  there  are  about  300  mols.  of  water  to  one  mol.  of  silicon 
dioxide,  and  only  about  two-thirds  of  this  water  may  be  removed  by 
rubbing  and  by  filtration.  Some  gels  rich  in  water  possess  elasticity 
which  tends  to  prevent  division.  They  may  be  rubbed  into  clumps 
that  form  larger  lumps  when  a  portion  of  the  water  flows  off.  The 
running  together  of  the  smaller  lumps  recalls  the  behavior  of  viscous 
liquids,  and  the  comparison  of  gel  formation  to  the  separation  of  two 
liquids  by  Butschli,  van  Bemmelen,  and  Quincke  is  thus  far  justified. 
However  the  assumption  that  the  ultramicrons  of  gels  or  sols  of 
lyophile  colloids  are  themselves  liquid,  is  no  more  reasonable  than  to 
insist  that  the  molecules  of  liquids  are  liquid,  or  that  the  molecules  of 
gases  are  gas.  Any  conclusion  with  regard  to  the  particles  themselves 

*  M.  de  Senarmont:  Annales  de  Chim.  et  de  Phys.  (3),  32,  129-175  (1851). 
t  K.  v.  Chrustschoff:  N.  Jahrb.  f.  Min.  usw.,  1,  205  (1887). 
f  W.  Bruhns:  N.  Jahrb.  f.  Min.  usw.,  2,  62-65  (1889). 


138 


CHEMISTRY  OF  COLLOIDS 


that  is  based  on  aggregates  is  unjustifiable.     The  question  of  the  state 
of  aggregation  of  the  ultramicrons  remains  undecided. 

Because  of  the  applicability  of  Boltzmann's  *  gas  theory,  which  con- 
siders molecules  to  be  completely  elastic  material  particles  incapable 
of  much  deformation,  and  because  according  to  van  der  Waals  f  the 
properties  of  molecules  must  be  compared  with  those  of  solids,  it  seems 
necessary  to  assume  that  the  -larger  ultramicrons  of  a  solid  are  them- 
selves solid.  The  so-called  liquid  properties  of  gels  rich  in  water  would 
therefore  be  explained  by  the  assumption  that  the  ultramicrons  are 
surrounded  by  water  layers  and  have  a  certain  free  path  and  motion. 
This  would  make  possible  the  surface  tension  effects  that  according  to 
Quincke  i  arise  from  the  attraction  of  the  electrically  neutral  particles 
for  one  another. 

The  water  that  is  pressed  out  from  the  gel  contains  only  a  small 
portion  of  the  colloid  while  the  so-called  viscous  or  oily  liquid  contains 
the  remainder.  These  two  " liquids"  will  be  discussed  later. 

The  Solidification  of  the  Gel  of  Silicic  Acid  on  Evaporation.  —  The 
dryer  the  gel  becomes  the  more  difficult  it  is  to  press  out  water.  Table  § 
22  gives  an  idea  of  the  changes  that  the  gel  undergoes  during  dehy- 
dration. 

TABLE  22 


Number  of  mols. 
water  to  one  mol. 
Si02. 

Properties  of  the  hydrogel. 

40-30 
20 
10 

6 

Gel  may  be  cut 
Fairly  stiff 
Brittle 
jMay  be  pulverized,   powder 
i     apparently  dry 

The  Change  in  Turbidity  and  in  Volume  During  Dehydration. If — • 
When  more  water  is  driven  off  the  volume  decreases  to  a  characteristic 
point,  called  by  van  Bemmelen  the  transition  point,  and  designated 
by  0.  From  this  on  the  volume  remains  constant.  Further  dehydra- 
tion causes  changes  in  the  optical  properties.  The  previously  clear  gel 
becomes  turbid  and  chalk  white  (transition) .  The  residue  becomes 
clear  again  when  the  water  content  goes  below  1  mol.  The  change  in 

*  L.  Boltzmann:  Vorlesungen  uber  Gastheorie,  34.     Leipzig  (1896). 

f  J.  D.  van  der  Waals:  Die  Kontinuitat  des  gasformigen  und  fliissigen  Zustandes, 
page  34.  Leipzig  (1899). 

t  G.  Quincke:  Drudes  Annalen  d.  Phys.  (4),  9,  793-836,  969-1045  (1902);  10, 
478-521,  673-703  (1903). 

§  J.  M.  van  Bemmelen:  Zeit.  f.  anorg.  Chemie,  69,  225-247  (1908). 

H"  J.  M.  van  Bemmelen:  Zeit.  f.  anorg.  Chemie,  18,  98-146  (1898). 


COLLOIDAL  OXIDES 


139 


volume  can  be  seen  in  Table  23.*    The  addition  of  water  causes  little 

change  of  volume. 

TABLE  23 


Water  content  in  mols. 

Volume. 

122 

29 

75.7 

18 

45.2 

11 

23.2 

4 

11.3 

3 

2.8 

.1 

2.2 

0.86 

1.7 

0.75 

Transition 

1.0 

0.73 

0.39 

0.73 

0.3 

0.73 

Organogels  of  Silicic  Acid 

Important  for  their  qualitative  determination  is  the  remarkable 
property  of  hydrosols  that  allows  the  water  to  be  replaced  by  alcohol, 
acetic  acid,  glycerin,  concentrated  sulfuric  acid,  etc.,  and  these  liquids 
again  by  water  without  any  appreciable  change  in  the  elasticity  or 
transparency.  Graham  f  was  able  to  replace  the  water  with  alcohol, 
and  thus  change  the  hydrogel  into  a  weakly  opalescent  alcogel  having 
a  volume  almost  identical  with  that  of  the  hydrogel.  The  composition 
of  the  alcogel  was  as  follows:  per 

Alcohol. 88. 13 

Water 0.23 

Si02 11 .04 

By  putting  the  alcogel  into  water  the  alcohol  was  replaced  by  the 
water  without  any  marked  change  taking  place  in  the  properties  of  the 
gel.  The  same  thing  holds  for  sulfuric  acid,  nitric  acid,  formic  acid, 
etc.  Van  Bemmelen  obtained  an  acetogel  containing, 

1        mol.  Si02 
0.25mol.  H2O 
21 . 7    mols.  acetic  acid. 

The  fact  that  the  water  may  be  replaced  by  other  substances  as  sol- 
vents without  any  marked  change  in  the  character  of  the  gel,  points 

*  J.  M.  van  Bemmelen:  Zeit.  f.  anorg.  Chemie,  59,  225-247  (1908). 
t  Th.  Graham:  Poggendorffs  Annalen,  123,  529  (1864). 


140  CHEMISTRY  OF  COLLOIDS 

clearly,  as  van  Bemmelen  has  noted,  to  the  assumption  that  the  water 
is  not  there  as  hydrate  chemically  combined,  but  is  adsorbed  water 
that  fills  the  spaces  between  the  ultramicrons.* 

The  Structure  of  Silicic  Acid  Gels  as  Revealed  by  the  Microscope 
and  Ultramicroscope.  —  The  structure  under  the  ultramicroscope  of 
silicic  acid  gels  is  more  pronounced  f  than  that  of  gelatin  of  the  same 
concentration.  Silicic  acid  gels  having  a  concentration  of  1  to  3  per 
cent  are  most  suitable  although  gels  of  higher  concentration  still  exhibit 
discontinuity.  These  latter  are  very  similar  to  gels  of  gelatin  having 
a  concentration  of  1  to  2  per  cent:  The  structure  of  these  is  more 
granular  than  that  of  more  dilute  silicic  acid  gels.  The  granular  appear- 
ance is  due  doubtless  to  the  joining  of  the  ultramicrons  together. 

As  in  the  case  of  gelatin,  in  very  dilute  solutions  where  the  concentra- 
tion of  the  colloidal  silicic  acid  is  about  one-half  of  one  per  cent,  floccu- 
lent  precipitates  are  formed  which  contain  large  spaces  filled  with 
water.  The  constituents  of  the  gel  are  no  longer  able  to  form  a  mass 
in  which  the  ultramicrons  are  uniformly  distributed.  As  the  water 
content  becomes  less  and  less  the  spaces  filled  with  water  shrink,  the 
particles  of  the  colloid  draw  closer  together,  and  it  is  thus  clear  why 
the  gels  from  concentrated  solutions  exhibit  a  denser  structure  than 
those  from  more  dilute  solutions.  That  this  is  true  to  fact  is  shown  by 
observations  under  the  ultramicroscope  during  the  formation  of  a  gel,* 
and  also  from  the  polarization  effects. 

The  Structure  of  Dehydrated  Gels.  —  Butschli  J  has  observed  a 
honeycomb  structure  in  many  dried  gels  of  silicic  acid.  The  structure 
is  not  apparent  until  the  spaces  are  filled  with  a  suitable  liquid  such  as 
chloroform  or  the  oil  of  cedar.  As  soon  as  a  portion  of  the  liquid  is 
evaporated  the  structure  becomes  discernible.  Butschli  explains  this 
on  the  assumption  that  the  walls  are  too  thin  to  be  seen  until  the  liquid 
has  caused  them  to  distend  somewhat.  If  the  liquid  is  partially  evapo- 
rated off  the  thickened  walls  become  visible.  When  the  spaces  are 
again  filled  with  liquid  the  walls  disappear  because  they  are  surrounded 
by  a  homogeneous  medium.  Butschli  has  determined  the  interstices 
to  have  a  diameter  of  1  to  1.5  JJL  and  the  thickened  walls  0.2  to  0.3  /*, 
and  therefore  the  walls  before  they  are  treated  with  the  liquid  must 
be  much  finer  than  this.  The  work  and  measurements  of  Butschli 
have  been  accepted  by  many  investigators.  To  the  author,  however, 

*  G.  Tschermak:  Zeit.  f.  phys.  Chemie,  53,  349-367  (1905).  G.  Tammann: 
Zeit.  f.  anorg.  Chemie,  71,  375  (1911). 

t  W.  Bachmann:  Inaug.-Diss.  Gottingen  (1911).  Zeit.  f.  anorg.  Chem.,  73, 
125-172  (1911). 

J  O.  Butschli:  Untersuchungen  iiber  die  Mikrestruktur  kiinstlicher  und  natiir- 
licher  Kieselsauregallerten.  Heidelberg  (1900). 


COLLOIDAL  OXIDES  141 

it  appears  difficult  to  conceive  that  spaces  filled  with  air,  and  having 
a  diameter  of  1  to  1.5  /*,  can  exist  in  a  clear  gel,  as  these  spaces  are 
enormous  compared  with  the  ultramicrons  themselves.  For,  if  the  index 
of  refraction  of  the  disperse  phase  and  the  disperse  medium  differ  at  all, 
disperse  systems  having  particles  1  to  1.5  ^  are  always  very  turbid  even 
when  the  concentration  is  low.  Such  is  the  case  of  kaolin  suspensions, 
clay  suspensions,  and  oil  emulsions.  A  much  greater  subdivision  is 
necessary  in  order  to  have  a  clear  colloidal  solution.  A  Si02  foam 
filled  with  air  where  the  spaces  have  dimensions  of  1  M  and  over  must 
appear  white  and  opaque  because  of  the  refraction  and  reflection  of 
light,  and  manifest  heterogeneity  under  the  ultramicroscope.  The 
observations  reveal,  however,  that  the  dried  gels  often  contain  submi- 
crons  and  often  are  quite  empty  from  an  optical  standpoint.  This 
points  strongly  to  a  much  finer  structure  than  Biitschli  has  ascribed 
to  them. 

The  following  experiment  was  carried  out  by  the  author.  A  clear 
dry  gel  having  very  faintly  discernible  submicrons  was  treated  with 
the  vapors  of  benzol,  whereby  it  adsorbed  37  per  cent  of  its  own  weight 
of  the  latter  and  remained  optically  clear.  Under  the  ultramicroscope 
the  benzol  was  allowed  to  evaporate,  and  the  following  process  was 
seen  to  take  place.  First  a  faint  light  cone  appeared  that  gradually 
became  more  pronounced.  Then  submicrons  appeared  that  became 
so  bright  that  they  illuminated  the  neighboring  particles.  These  sub- 
microns  could  not  be  counted.  Finally  the  light  cone  gradually  became 
weak  again.  The  submicrons  were  very  thickly  distributed  and  were 
quite  like  those  in  ruby  glass.  There  were  also  particles  just  within 
the  resolving  power  of  the  ultramicroscope.*  Here,  as  in  the  case  of 
ruby  glass,  heterogeneity  could  be  observed,  and  forms  composed  of  nu- 
merous amicrons  piled  up  could  be  discerned.  The  light  was  linear 
polarized  and  the  ray  could  be  cut  off  by  turning  the  nicol.  This  is 
another  evidence  for  the  fine  state  of  subdivision. 

The  Transition.  —  The  phenomena  of  the  transition  may  be  ex- 
plained on  the  following  grounds.  The  mixture  of  air  and  silicic  acid 
is  optically  clear  because  both  the  particles  and  the  spaces  between 
them  are  amicroscopic.  Only  a  few  of  the  thicker  masses  of  amicrons 
affect  the  light  as  submicrons,  and  can  therefore  be  seen  in  the  dried 
preparation.  The  coefficient  of  refraction  of  the  amicroscopic  mixture 
of  benzol  and  silicic  acid  lies  between  that  of  benzol  and  that  of  silicic 
acid.  Likewise  the  coefficient  of  refraction  of  the  mixture  air  and  silicic 
acid  lies  between  that  of  the  two  substances.  During  the  evaporation 
of  the  benzol  countless  tiny  spaces  are  formed,  filled  with  benzol  vapor 

*  H.  Siedentopf  und  R.  Zsigmondy:  Drudes  Annalen  d.  Phys.  (4),  10,  1-39  (1903). 


142 


CHEMISTRY  OF   COLLOIDS 


CjAirinGeL 


or  with  a  mixture  of  air  and  benzol  vapor.  The  spaces  gradually 
become  larger  and  we  have  a  gas-silicic  acid  mixture  with  a  coefficient 

differing  from  that  of  the  benzol- 
silicic  acid  mixture.*  Light  is  now 
refracted  and  when  the  gas  filled 
spaces  become  large  enough  sub- 
microns  may  be  detected.  Fig.  22 
represents  a  schematic  view  of  the 
process. 

The  gas  filled  spaces  in  the  silicic 
acid  skeleton  grow  in  an  irregular 
manner,  and  when  they  become 
large  enough  the  structure  appears 
honeycombed.  According  to  this 

FIG.  22.     Submicroscopical  air  bubbles  Point  of  view  the  walls  surrounding 
in  the  gel  of  silicic  acid.  the    spaces    consist    of    silicic    acid 

soaked  in  benzol,  and  the  spaces  of 

Butschli  as  gas  filled  silicic  acid.  An  observation  by  Bachmann  f  cor- 
roborates this  idea.  With  a  silicic  acid  gel  soaked  in  benzol  he  saw, 
in  the  ultramicroscope  during  the  evaporation,  a  constantly  changing 
picture.  Large  numbers  of  submicrons  appeared  and  disappeared  again 
under  the  eye  of  the  observer.  Blocks  of  ultramicrons  were  formed 
and  were  continuously  torn  asunder.  This  is  to  be  attributed  to  the 
withdrawal  of  the  benzol  and  the  changing  distribution  of  liquid  and 
gas  in  the  amicroscopical  canals.  With  complete  evaporation  of  the 
benzol  the  system  loses  its  coarser  heterogeneity  and  the  original  weak 
dispersion  of  the  light  is  again  established,  which  does  not  correspond 
to  the  finest  structure  of  the  gel. 

From  what  has  been  said  it  may  be  concluded  that  Biitschli's  honey- 
comb is  not  the  true  structure  of  the  gel,  but  on  the  contrary  is  a 
coarser  heterogeneity  due  to  the  accumulations  of  liquid  in  a  silicic 
acid  conglomerate,  which  is  permeated  with  amicroscopical  spaces. 

Dehydration  of  Silicic  Acid  by  Periodic  Reduction  of  Pressure 

Van  Bemmelen  {  has  carried  out  elaborate  and  important  experi- 
ments with  regard  to  the  behavior  of  silicic  acid  hydrogels  during  de- 
hydration. He  put  the  hydrogel  into  desiccators,  36  in  number,  con- 

*  D.  Brewster:  Philos.  Transact.,  11,  283  (1819).  Schweiggers  Journ.  f.  Chem. 
u.  Phys.,  29,  411-429  (1820). 

f  W.  Bachmann:  Zeit.  f.  anorg.  Chemie,  73,  165  (1911). 

t  van  Bemmelen:  Zeit.  f.  anorg.  Chemie,  13,  233-356  (1897);  69,  225-247 
(1908);  62,  1-23  (1909).  J.  S.  Anderson:  Zeit.  phys.  Chemie,  88,  191-228. 


COLLOIDAL  OXIDES 


143 


taining  varying  concentrations  of  sulfuric  acid,  and  in  this  manner 
graded  the  aqueous  vapor  pressure  from  0  to  12.7  mm.  The  hydrogel 
was  first  put  over  the  most  dilute  sulfuric  acid  and  left  until  the  weight 
ceased  to  shrink.  This  was  continued  down  the  row  of  desiccators 
until  the  last  contained  the  most  concentrated  sulfuric  acid.  The  re- 
sults showed  that  the  dehydration  was  not  the  same  for  all  gels,  but 
depended  upon  the  method  of  preparation  and  the  previous  history. 
The  process  is  shown  schematically  in  Fig.  23. 

The  vapor  pressure  decreases  along  the  curve  A$  while  the  volume 
simultaneously  decreases.  In  point  0,  the  transition  point,  the  curve 
breaks  suddenly.  Along  the  line  OOi  water  is  given  up  by  the  gel  at 
almost  constant  pressure.  The  volume  of  the  hydrogel  remains  almost 


_  Pres8ure_of_the_satui^ted_water  vapor_  _        O$ 


O 


(Transition) 
15* 


OQ  Water  content  of  gel3 

FIG.  23.    Water  content  of  the,  gel. 

constant  in  spite  of  the  loss  of  water.  Along  the  line  OOi  a  peculiar 
phenomenon  takes  place;  the  previously  clear  gel  becomes  turbid, 
clears  up  again,  and  at  Oi  is  completely  clear.  Further  dehydration 
follows  the  line  Aa;  the  last  stages  are  possible  only  by  the  employment 
of  red  heat.  It  is  remarkable  that  the  dehydration  along  the  curve  Aft 
is  quite  irreversible,  and  if  water  is  now  added  a  new  curve  is  followed, 
viz.,  curve  Zy.  On  the  contrary  the  curve  OOi  represents  a  completely 
reversible  process  at  every  point.  Curve  ZaZp  is  the  hydration  proc- 
ess, probably  an  example  of  hysteresis.  If  the  gel  is  again  dehydrated 
curve  Zy  is  followed. 

From  the  fact  that  the  point  0  represents  two  mols.  of  water  to  one 
mol.  of  silicic  acid,  and  that  point  03  represents  one  to  one,  it  may  be 
concluded  that  the  process  is  one  of  decomposition  of  a  hydrate.  The 
vapor  pressure  at  0  is  the  vapor  pressure  of  the  orthohydrate  and  the 


144  CHEMISTRY  OF  COLLOIDS 

turbidity  is  caused  by  the  appearance  of  a  new  phase.  This  point  of 
view  is  contradicted,  however,  by  several  curves  of  van  Bemmelen.  He 
found  that  the  points  0  and  0\  did  not  correspond  in  the  majority  of 
cases  to  2  mols.  and  1  mol.  of  water  respectively,  but  that  the  former 
might  lie  between  1.5  and  3,  while  the  latter  usually  lay  between  0.5 
and  1.  Van  Bemmelen  objected  to  the  assumption  of  hydrates  from 
the  above  considerations.  To  the  author  it  seems  much  more  reason- 
able to  assume  that  the  dehydration  along  the  line  00}  is  to  be  at- 
tributed to  the  emptying  of  the  spaces  in  the  gel,  because  they  may  be 
filled  by  many  other  liquids  such  as  alcohol,  benzol,  benzine,  etc.,  and 
still  the  phenomenon  of  turbidity  occurs  during  the  evaporation  of  the 
liquid  in  question. 

An  important  question  in  this  connection  is  the  size  of  the  spaces 
concerning  which  considerable  has  already  been  said.  From  the  evi- 
dence it  seems  probable  that  they  must  be  very  small.  That  they 
must  be  connected  with  one  another  is  indicated  by  the  rapidity  with 
which  a  liquid  will  completely  permeate  the  entire  mass  of  the  gel.  If 
we  assume  that  these  spaces  are  tiny  capillaries  then  the  dehydration 
must  obey  the  laws  of  capillarity,  and  from  this  some  idea  of  the  size 
of  the  capillaries  may  be  obtained. 


Here  PQ  is  the  saturation  pressure  over  the  level  liquid. 

PB  is  the  saturation  pressure  over  the  center  of  the  meniscus. 
TAB  is  the  surface  tension. 
gA  is  the  density  of  the  liquid. 
gB  is  the  density  of*  the  vapor. 

According  to  the  formula,  a  lowering  of  the  vapor  pressure  of  6  mm. 
would  correspond  to  a  capillary  diameter  of  5  MM-  A  capillarity  of 
this  degree  of  fineness  would  agree  with  the  optical  properties  of  the 
hydrogel. 

Application  of  the  Theory  of  Capillarity  to  the  Process  of 
Dehydration 

I.  Let  us  now  see  if  van  Bemmelen's  curves  can  be  accounted  for 
on  the  basis  of  the  laws  of  capillarity.  In  order  to  understand  this 
better  it  is  perhaps  necessary  to  recall  some  of  the  theory  of  capil- 
larity.* Assume  a  liquid  touching  a  capillary  tube  forms  a  meniscus 
concave  upwards,  as  represented  in  Fig.  24.  Because  of  the  surface 
tension  on  the  liquid  it  rises  in  the  capillary.  The  pressure  upward, 

*  B.  E.  Riecke:  Lehrbiichern  der  Physik  (3  AufL),  1.    Leipzig  (1905). 


COLLOIDAL  OXIDES  145 


represented  by  2  irrT,  is  equal  to  the  weight  of  the  liquid  raised, 
where  T  is  the  surface  tension,  h  is  the  height  of  the  column  of  liquid, 
and  o-  is  the  specific  gravity  of  the  liquid.*    From  this  it  follows  that 

2T 

fi  —  —  •  • 
ra 

The  formula  holds  only  for  a  liquid  completely  surrounded  by  very 
small  cylindrical  capillaries.  Then  the  radius  of  the  curvature  of  the 
liquid  gas  surface  is  equal  to  the  radius  of  the  tube.  The  theoretical 
height  of  water  in  capillary  tubes  having  a  diameter  of  5  w  would  be 
several  kilometers.  If  the  radius  of  curvature  is  very  small  and  the 
height  correspondingly  great  the  liquid  would  evaporate  under  a  pres- 
sure considerably  less  than  that  which  it  would  have  from  a  plane  sur- 
face. The  lowering  of  the  vapor  pressure  may  be  calculated  from  the 
formula  given  on  page  144.f 

II.  If  the  liquid  is  not  completely  surrounded  the  angle  that  the 
liquid  makes  with  the  solid  varies  from  0  upwards,  and  the  radius 
of  curvature  would  be  greater  than  the  radius  of  the  tube.     Conse- 
quently the  height  must  be  less  and  the  vapor  pressure  greater.     This 
is  of  importance  in  connection  with  hydration  curves. 

III.  Wherever  the  meniscus  is  concave  toward  the  gas,  surface  ten- 
sion exerts  a  pull  on  the  liquid.     The  result  of  this  pull  is  the  rise  of 
the  liquid  in  the  capillary.     A  corresponding  and  opposite  pressure  is 
exerted  on  the  capillary.    Where  the  capillary  is  vertical  this  pressure 
is  equal  to  the  weight  of  the  liquid  raised. 

What  has  been  said  will  be  sufficient  to  make  the  dehydration  curves 
or  isotherms  clear.  Let  us  consider  gel  in  the  curves  OOi  00  02  0.  We 
may  imagine  the  gel  in  point  0  to  contain  numberless  tiny  capillaries 
filled  with  liquid.  The  surface  of  the  liquid  in  each  capillary  is  a  half 
sphere  with  the  concave  surface  toward  the  gas,  and  a  very  small  radius 
of  curvature  because  of  the  miniature  dimensions  of  the  capillaries. 
According  to  (1)  such  an  arrangement  causes  an 
enormous  pull  on  the  liquid  in  the  direction  of  the 
upright  arrows  in  Fig.  24  and  a  corresponding  pres- 
sure in  the  opposite  direction  on  the  walls.  The 
latter  would  press  the  amicrons  in  the  walls  close  FlG  24. 

together  and  cause  a  contraction  of  the  gel,  while 
the  former  would  favor  the  escape  of  tiny  air  bubbles  from  the  interior 
of  the  gel.     These  bubbles  would  therefore  cause  the  turbidity  in  the 
transition  point. 

*  B.  E.  Riecke:  Lehrbuchern  der  Physik  (3  Aufl.),  1.     Leipzig  (1905). 
t  Lord  Kelvin:  Proc.  Roy.  Soc.,  7,  63-68  (1870). 


146 


CHEMISTRY  OF  COLLOIDS 


(5)  The  state  of  the  gel  in  0  is  characterized  by  a  dilatation  of  the 
enclosed  liquid  and  a  compression  of  the  surrounding  silicic  acid. 

Both  the  compression  of  the  silicic  acid  and  the  dilatation  of  the 
liquid  are  dependent  upon  the  radius  of  curvature  of  the  liquid  surface, 

^  o'  Fig.  25.     Both  are  large  when  R  is 

small,  and  both  approach  zero  when 
R  is  approaching  infinity,  that  is 
when  the  surface  of  the  liquid  is  a 
plane.  On  the  addition  of  a  very 
small  amount  of  water  the  radius  of 
curvature  would  be  greatly  increased, 
and  the  surface  tension  relations  in 
the  gel  would  be  changed  a  great 
deal.  In  other  words  there  would 
be  a  decrease  in  the  surface  tension, 
in  the  compression  of  the  silicic  acid, 


FIG.  25.    R  <  R'  and  p  >  p'. 


and  in  the  dilatation  of  the  liquid.  The  liquid  would  therefore  con- 
tract, the  gel  would  expand  somewhat,  and  take  up  a  corresponding 
amount  of  water.  These  relations  are  of  importance  in  the  consider- 
ation of  the  curves  Zy  and  the  addition  of  water  from  0%  to  03. 

Dehydration  of  the  Hydrogel 

The  fact  that  the  water  can  be  evaporated  at  normal  vapor  tension 
until  the  relations  are  about  6  mols.  water  to  1  mol.  silicic  acid  would 
indicate  that  this  water  is  loosely  joined,  or  can  be  easily  pressed  out. 
From  the  reduction  of  the  vapor  tension  for  quantities  less  than  6  to  1 
it  may  be  concluded  that  the  meniscus  begins  to  be  concave  toward 
the  gas  phase  at  this  point.  From  the  reduction  of  the  vapor  tension 
along  curve  Ap  we  cannot  calculate  the  diameter  of  the  capillaries 
because  the  volume  of  the  gel  varies  continuously  until  0  is  reached. 
The  formula  given  above  is  not  applicable  until  the  gel  ceases  to  con- 
tract by  further  dehydration.  These  conditions  obtain  at  the  transi- 
tion point  so  that  the  formula  may  be  used  to  calculate  the  size  of  the 
capillaries. 

Dehydration  Along  the  Line  OOi.  —  Biitschli  has  observed,  and  it 
has  been  corroborated  by  work  with  the  ultramicroscope,  that  the  de- 
hydration does  not  go  on  from  the  outside  toward  the  interior,  but  that 
holes  are  formed  in  the  interior  free  of  liquid.  It  may  be  assumed 
that  at  point  0  the  water  on  the  surface  capillaries  has  a  radius  of  curv- 
ature corresponding  to  the  vapor  tension,  as  shown  in  Fig.  26.  Accord- 
ing to  3,  page  145,  this  would  cause  the  evolution  of  gas  in  the  interior 
of  the  mass  much  like  the  effervescence  under  reduced  pressure  of  a 


COLLOIDAL  OXIDES  147 

liquid  saturated  with  gas.    The  horizontal  position  of  the  curve  OOi 
would  indicate  that  the  radius  of  curvature  of  the  capillaries  in  the 
interior  is  very  little  greater  than  those  on  the 
surface.     In  point  0\  the  capillaries  are  almost 
empty  and  the  curve  0i0o  represents  the  with- 
drawal of  the  adsorbed  water,  or  possibly  water 
of  hydration.*     In  the  latter  case  it  would  be 
necessary  to  assume,  as  Tammann  f  has  done  for 
the  water  contents  of  many  crystallized  minerals, 
that  the  hydrate  and  the   anhydride   formed  a 
solid  solution.     As  already  intimated,  the  dehy-  FIG.  26.    A,  Surface  of 
dration  along  this  curve  is  quite  reversible  and        gel.    B,  Interior, 
the  equilibria  are  quickly  established.  { 

The  Addition  of  Water  to  the  Gel.  Filling  the  Capillaries.  —  The 
variation  of  the  curve  0i02  from  OiO  may  be  explained  on  the  following 
grounds.  It  is  well  known  that  a  liquid  will  not  rise  to  the  same  height 
in  single  tubes  as  it  will  if  the  tubes  form  a  network.  In  other  words 
the  radius  of  curvature  is  greater  in  the  single  tubes.  But  a  greater 
radius  of  curvature  corresponds  to  higher  vapor  tension.  From 
this  point  of  view  the  filling  of  the  tubes  occurs  under  a  greater  vapor 
tension  than  did  the  dehydration,  because  all  the  tubes  are  filled  with 
liquid  .in  the  latter  case  as  long  as  the  water  suffices.  This  seems 
to  be  the  simplest  explanation  of  the  hysteresis  in  the  field  00A0. 
The  water  does  not  cover  all  the  tubes  at  first;  the  radius  of  curvature  is 
greater,  and  therefore  the  vapor  tension  is  greater  during  the  addition 
of  water  than  during  the  dehydration.  The  air  in  the  mass  doubtless 
prevents  the  water  from  completely  filling  the  network  of  tubes.  Other 
explanations  of  the  hysteresis  are  possible.  § 

In  point  02  the  capillaries  are  again  filled,  but  the  vapor  tension 
is  greater  than  it  is  at  the  transition  point.  So  also  is  the  radius  of 
curvature  and  this  corresponds  to  a  lessened  pressure  on  the  gel.  As  a 
consequence  the  gel  can  take  up  more  water  than  it  can  at  0  and  the 
point  02  therefore  lies  somewhat  to  the  right  of  0. 

The  position  of  the  ^/-curves  may  be  explained  on  the  same  grounds, 
viz.,  the  change  in  the  radius  of  curvature.  The  gel  is  not  capable  of 
distension  except  within  the  limits  of  elasticity.  In  point  03  the  vapor 
tension  in  the  gel  is  equal  to  that  of  water  at  the  same  temperature, 
consequently  the  water  in  the  gel  must  have  a  plane  surface  toward  the 

*  H.  Freundlich:  Kapillarchemie,  491. 
t  G.  Tammann:  Zeit.  f.  phys.  Chemie,  27,  323-336  (1898). 
j  van  Bemmelen:  Zeit.  f.  anorg.  Chemie,  13,  258  (1897). 
§  H.  Freundlich:  KapUlarchemie,  486  ff. 


148  CHEMISTRY  OF  COLLOIDS 


gas  phase.  Along  the  curve  OzOs  there  is,  therefore,  no  pressure  on 
the  gel.  This  same  explanation  would  hold  for  all  other  Zy  curves 
and  the  almost  straight  line  formation  indicates  the  correctness  of  the 
underlying  assumptions. 

Irreversible  Changes  of  State.  —  The  dehydration  of  the  hydrogel 
of  silicic  acid  along  the  curve  Ap  presents  two  important  questions. 

1.  Why  is  the  reaction  along  the  curve  Ap  not  reversible? 

2.  What  is  the  nature  of  the  irreversible  change  of  structure,  along 
the  curve  Ap?    As  will  be  shown  later  van  Bemmelen  has  demonstrated 
that  a  change  in  the  position  and  in  the  length  of  the  curve  00  is  brought 
about  by  ageing  of  the  gel  under  constant  vapor  pressure.     See  Fig.  27.* 

With  regard  to  the  first  question  there  are  several  possibilities,  and 
these  require  further  careful  study  before  comprehensive  discussion  is 
possible.  The  second  seems  to  present  more  vulnerable  points  of 


12.7 
10 


O0     0.5                1.0                1.5                2.0                2.5                3.0  3.5 
Gel  A  (fresh) 8  months  old     2^  years  old 

FIG.  27.    Effect  of  ageing. 

attack.  In  considering  the  change  of  structure  with  the  lapse  of  time, 
two  reactions  that  play  an  important  part  in  all  colloidal  chemistry 
must  not  be  lost  sight  of,  viz.,  the  union  of  the  particles,  and  the 
growth  of  the  larger  particles  at  the  expense  of  the  smaller,  possibly 
also  the  growth  by  crystallization.  Both  must  lead  to  an  enlargement 
of  the  interstices  between  the  particles  and  to  a  solidification  of  the  gel 
residue,  by  which  the  total  volume  of  the  gel  remains  almost  constant, 
under  constant  vapor  tension, 

Influence  of  Ageing.  —  If  crystallization  or  other  processes  that 
enlarge  the  spaces  between  the  particles,  occur  during  the  ageing  of  the 
gel,  the  vapor  tension  of  such  aged  gels  must  be  higher  than  that  of 
the  newly  prepared.  This  can  be  seen  from  Fig.  27.  The  older  the  gel, 
the  higher  is  the  vapor  pressure  at  the  transition  point.  The  ageing 
of  the  particular  gel  investigated  took  place  at  the  saturation  point  of 
aqueous  vapor. 

*  van  Bemmelen:  Zeit.  C.  anorg.  Chemie,  13,  345  (1897). 


COLLOIDAL  OXIDES  149 

The  Volume  of  Air  in  the  Spaces  Between  the  Particles 

Van  Bemmelen  *  found  the  following  specific  gravities  for  dried  gels. 

A  (fresh),  No.  107 1.17 

A,  6  months  old,  No.  105 1.05 

A,  5  years  old,  No.  106 0.9 

From  this  and  from  the  specific  gravity  of  the  gel  substance  itself,  he 
calculated  the  volume  of  the  spaces,  evaluated  against  1  volume  of  the 
gel  substance. 

A  (fresh) 0.71 

A,  6  months  old  . 0,94 

Aj  5  years  old 1 . 25 

The  volume  of  the  adsorbed  air  is  considerably  greater  than  that  of 
the  spaces,  from  which  it  follows  that  the  air  must  be  condensed.  1 
volume  of  the  interspace  contained  the  following  volumes  of  air  cal- 
culated at  normal  temperature  and  pressure. 

A  (fresh) 4.2 

A,  6  months  old 2.65 

A,  5  years  old 2.0 

These  figures  indicate  that  a  decrease  in  the  total  surface  of  the 
particles  must  occur  during  the  ageing,  because  the  smaller  the  sur- 
face exposed  the  less  gas  could  be  adsorbed.  At  the  same  time  the 
interspaces  are  becoming  larger,  and  this  would  go  to  show  that  the 
ultramicrons  are  growing  at  the  expense  of  the  smaller  in  a  manner 
similar  to  a  crystallization  process. 

Effect  of  Ignition 

A  low  red  heat  for  a  short  time  left  the  gel  in  such  a  state  that  the 
dehydration  curve  was  very  similar  to  that  of  the  unheated  gel  except 
that  the  interval  of  constant  pressure  OOi  is  very  much  shorter.  Longer 
heating  had  the  same  effect  as  may  be  seen  from  Fig.  28,  taken  from 
the  article  by  van  Bemmelen.  f  Strong  ignition  robs  the  gel  of  its 
property  of  taking  up  water  to  any  large  extent. 

According  to  Biitschli  t  the  amount  of  salt  in  the  gel  (difficult  to 
remove  by  washing)  has  an  influence  on  the  changes  during  heating. 
While  salt-free  gel  did  not  change  very  much  in  the  interior,  those  con- 
taining salts  became  turbid  and  lost  their  power  of  taking  up  water 

*  van  Bemmelen:  Zeit.  f.  anorg.  Chemie,  18,  114-117  (1898). 

t  Ibid.,  13,  350. 

t  O.  Biitschli:  Untersuchungen  tiber  Mikrostruktur.  usw.,  337. 


150 


CHEMISTRY  OF   COLLOIDS 


easily.  A  honeycomb  structure  could  be  detected  in  some  of  these 
heated  gels.  From  the  curves  and  from  microscopical  observations 
one  may  conclude  that  because  of  unequal  heating  in  the  crucible  the 
particles  melt  together  in  the  spots  that  are  at  the  higher  temperature, 
and  that  traces  of  salt  are  favorable  to  this  reaction.  On  the  other 
hand  those  spots  that  are  not  heated  to  the  requisite  temperature,  or 
are  free  from  salt,  retain  their  original  structure,  and  at  .the  transition 
point  must  have  the  same  vapor  tension  as  they  had  before  the  heating. 
However  as  a  portion  of  the  original  pores  will  be  damaged  on  each 
heating  the  total  amount  of  water  that  the  gel  will  take  up  must  be 
less  than  before  the  heating.  This  would  account  for  the  fact  that 
the  line  00 1  becomes  shorter  as  the  heating  is  continued. 


10 


0.5  1.0  1.5  2.0 

JYesh GdAi  Short.Heafing Gel^nAgain  Heated 

— Gel  AI  Fresli-a£ter  Dehydration.  Heated-strongly  lOatumtea 

FIG.  28.    Effect  of  red  heat. 

Staining  the  Gel  of  Silicic  Acid 

Corresponding  to  its  large  surface  silicic  acid  gel  has  considerable 
power  of  adsorption,  and  this  can  be  most  easily  demonstrated  with  dye- 
stuffs.  Acid  dyestuffs  are  feebly  adsorbed  and  may  be  almost  com- 
pletely removed  by  washing.  Basic  dyestuffs,  on  the  contrary,  are 
adsorbed  to  a  large  degree,  and  the  reaction  is  practically  irreversible; 
that  is  the  dye  cannot  be  removed  by  washing.  To  explain  this  it 
would  be  natural  to  suppose  that  the  silicic  acid  formed  a  salt  with 
the  basic  dye.  Opposed  to  this  point  of  view  is  the  fact  that  charcoal 
and  many  other  substances  behave  similarly  toward  acid  and  basic 
dyestuffs.  For  instance,  Fuchsin,  Methyl  Violet,  and  Malachite  Green 
are  sometimes  adsorbed  to  give  intensive  colors  that  cannot  be  re- 
moved by  washing,  while  acid  dyestuffs  may  be  easily  removed  by 
this  process. 


COLLOIDAL  OXIDES  151 

Tschermak  *  has  performed  some  experiments  on  the  staining  of 
gels  obtained  from  the  decomposition  of  various  mineral  silicates  and 
found  that  the  moist  gel  did  not  adsorb  so  well  as  the  dry.  He  further 
demonstrated  that  silicic  acid  from  different  sources  adsorbed  in  a 
varying  manner,  depending  upon  the  origin  and  method  of  prepara- 
tion. Kaolin,  studied  by  Suida,f  gave  results  in  accord  with  gels  from 
minerals.  This  is  probably  due  to  the  fact  that  the  surface  of  the 
kaolin  particles  is  covered  with  silicic  acid  gel,  because  the  reaction  is 
not  changed  if  kaolin  is  treated  with  hydrochloric  acid.  Again  if  the 
kaolin  is  etched  with  hydrofluoric  acid  so  that  the  surface  is  no  longer 
silicic  acid  but  rather  the  oxides  of  the  aluminium  group,  acid  dye- 
stuffs  are  adsorbed  just  as  in  the  case  of  aluminium  oxide.  According 
to  H.  Ambronn  {  silicic  acid  gel  becomes  brown  in  a  violet  solution  of 
iodine,  and  Kuster  has  made  use  of  this  reaction  as  a  test  for  the  pres- 
ence of  the  gel  in  plants. 

Reaction  of  the  Dehydrated  Gel  Towards  Hydrosols 

As  already  pointed  out  the  mass  of  a  silicic  acid  gel  is  permeated 
with  extraordinarily  fine  pores.  This  is  consistent  with  its  power  of 
becoming  wet  through  and  through  very  quickly  by  water.  It  also 
explains  why  crystalloids  pass  easily  through,  and  why  it  becomes 
colored  by  dyestuffs,  but  does  not  permit  the  passage  of  colloidal 
particles.  The  author  has  made  use  of  this  last  property  for  the 
purposes  of  ultrafilt ration. 

If  the  gel  is  thrown  into  fuchsin  solution  the  dye  is  soon  adsorbed. 
In  colloidal  solution  the  dry  gel  soaks  up  the  water  like  a  sponge,  leaving 
the  colloid  in  a  mass  on  the  surface.  A  piece  of  the  dry  gel  dipped 
into  a  10  per  cent  solution  of  colloidal  silver  and  immediately  removed 
has  a  highly  reflecting  silver  mirror  on  the  surface.  Benzopurple 
remains  on  the  surface  and  does  not  penetrate  any  but  the  larger  cracks. 
The  same  may  be  said  of  Carmine  and  colloidal  iron  oxide.  Because 
the  gel  bursts  into  smaller  pieces  during  these  experiments  it  is  wise  to 
follow  the  reaction  under  the  microscope,  using  a  magnification  of  20 
to  30  diameters.  The  excess  of  the  hydrosol  may  be  removed  by 
water  or  by  alcohol. 

A  series  of  interesting  reactions  may  be  observed  under  these  condi- 
tions. Generally  the  gel  breaks  up  into  smaller  pieces  in  the  first 
few  moments.  This  is  doubtless  due  to  the  pressure  to  which  the  air 
in  the  pores  is  subjected,  and  to  changes  of  surface  tension  in  the  gel. 

*  G.  Tschermak:  Zeit.  f.  phys.  Chemie,  53,  349-367  (1905). 

t  W.  Suida:  Sitzungsber  d.  Akad.  d.  Wiss.,  Wien,'  113,  lib,  725-761  (1904). 

t  E.  Kuster:  Ber.  d.  D.  Botan.  Ges,,  15,  136-138  (1897). 


152  CHEMISTRY  OF  COLLOIDS 

See  the  discussion  on  page  144,  on  capillarity.  The  water  spreads  into  the 
interior  rapidly  at  first,  and  more  slowly  toward  the  end.  As  this  goes 
on  more  tiny  explosions  take  place  with  the  accompanying  splitting  of 
the  gel,  or  air  bubbles  are  formed  at  tiny  cracks  along  the  surface  of 
the  particles.  Both  the  moistened  and  the  dry  gels  are  transparent, 
but  differ  in  the  coefficient  of  refraction.  As  a  consequence  the  reac- 
tion is  much  more  easily  followed.  The  gel  filled  with  air  appears  like 
an  air  bubble  surrounded  by  a  strongly  refracting  medium.  As  the 
bursting  of  the  gel  or  the  evolution  of  the  air  progresses  the  volume  of 
the  air  spaces  gradually  diminishes  and  finally  disappears.  Benzine  is 
even  more  suitable  for  the  purpose  because  of  its  smaller  surface  tension 
and  consequent  slower  penetration  into  the  mass.  There  is  here  very 
little  evidence  of  explosion  so  manifest  in  the  case  of  water. 

Occurrence  of  Silicic  Acid  in  Nature 

Silicic  acid  gel  in  a  more  or  less  hydrated  form  occurs  in  nature  as 
opal,  chalcedon,  agate,  hydrophane,  etc.,  and  in  the  vegetable  kingdom 
as  tabashir.  Hydrophane  and  tabashir  possess  a  great  similarity  to 
artificial  gels.  They  also  take  up  water  with  the  evolution  of  gas  (air) 
bubbles  and  are  more  or  less  transparent  according  to  the  water  con- 
tent. Sometimes  however,  they  are  chalk  white.  Change  of  the  water 
content  of  opal  causes  the  same  splitting  process  that  is  so  common  with 
the  artificial  gels.  Liesegang  *  has  shown  that  agate  has  probably 
been  produced  from  a  gel  of  silicic  acid  into  which  layers  of  iron  and 
other  salts  have  penetrated.  The  process  is  probably  similar  to  that 
on  which  his  observations  were  made,  viz.,  the  diffusion  of  silver  nitrate 
in  gelatin  containing  chromate. 

B.   Colloidal  Stannic   Acid 

The  Hydrosol  of  Stannic  Acid 

According  to  Graham  t  the  hydrosol  of  stannic  acid  may  be  made 
by  the  addition  of  alkali  to  solutions  of  stannic  chloride,  and  subse- 
quent dialysis,  or  by  the  dialysis  of  sodium  stannate  after  the  addi- 
tion of  hydrochloric  acid.  The  gels  thus  obtained  may  be  peptised  by 
further  dialysis.  Excesses  of  alkali  may  be  neutralized  by  the  addition 
of  iodine  solution.  According  to  the  same  investigator  colloidal  stannic 
acid  is  changed  on  heating  to  metastannic  acid.  E.  A.  Schneider  J 

*  R.  E.  Liesegang:  Centralbl.  f.  Min.  Geol.  usw.,  593-597  (1910);  497-507 
(1911). 

t  Th.  Graham:   Poggendorffs  Annalen,  123,  538  (1864). 
t  E.  A.  Schneider:  Zeit.  f.  anorg.  Chemie,  6,  82  (1894). 


COLLOIDAL  OXIDES  153 

prepared  the  hydrosol  in  a  similar  manner  except  that  he  employed 
ammonium  hydroxide  instead  of  sodium  or  potassium. 

The  author  has  succeeded  in  preparing  a  hydrosol  that  is  relatively 
free  from  electrolytes  without  the  inconvenience  of  dialysis,  by  the 
following  methods.*  A  current  of  air  is  conducted  through  a  very 
dilute  solution  of  stannous  chloride  until  oxidation  is  complete.  A 
precipitate  separates  out  that  may  be  freed  from  chloride  by  wash- 
ing and  decantation,  and  which  is  soluble  in  very  little  ammonium 
hydroxide.  The  excess  of  ammonia  is  easily  evaporated  on  boiling. 
A  still  more  simple  method  f  is  to  dilute  a  solution  of  stannic  chloride 
whereby  the  latter  is  hydyrolyzed  almost  to  completion.  This  gel  may 
be  washed  by  decantation  and  peptised  as  in  the  above  method.  After 
the  boiling  both  gels  contain  1  mol.  of  ammonia  to  20  to  30  mols. 
Sn02. 

The  hydrosols  are  very  stable,  almost  optically  clear  and  may  be 
kept  for  years  without  any  change  taking  place  in  the  appearance. 
That  the  appearance  is  deceptive,  however,  is  shown  by  the  fact  that 
the  protective  effect  on  colloidal  gold  gradually  lessens.  On  freezing 
leaf -like  crystals  are  formed  that  are  no  longer  soluble  in  water. 

As  in  the  case  of  all  other  hydrosols  that  of  stannic  acid  exhibits 
certain  exceptional  reactions.  Most  electrolytes,  such  as  sodium  chlo- 
ride, hydrochloric  acid,  potassium  hydroxide,  etc.,  precipitate  it  at 
once.  Toward  sodium  chloride  and  many  other  alkali  salts  hydrosols 
of  stannic  acid  differ  in  their  behavior  from  those  of  silicic  acid.  The 
latter  are  not  immediately  coagulated  but  on  the  contrary  fall  out 
slowly.  The  precipitation  with  potassium  hydroxide  or  sodium  chlo- 
ride is  reversible;  that  is  to  say  the  precipitated  stannic  acid  goes 
back  into  solution  as  soon  as  the  electrolyte  is  removed  by  washing. 
On  the  other  hand  the  coagulation  with  acids  is  irreversible,  and  the 
precipitating  agent  may  be  washed  out  completely  without  losing  any 
of  the  gel.  See  the  chapter  on  peptisation,  page  74. 

If  the  solution  is  boiled  too  long  the  colloidal  stannic  acid  becomes 
an  almost  insoluble  glassy  mass  that  cannot  be  again  peptised.  The 
author  noticed  that  an  aged  solution  on  long  boiling  gave  a  rubber-like 
viscous  stringy  mass  that  was  soluble  in  water.  Complete  dehydration 
destroys  the  soluble  properties  of  stannic  acid  gels  to  such  a  degree  that 
they  may  not  be  peptised.  On  electrolysis  the  stannic  acid  is  precipi- 
tated on  the  anode  in  the  form  of  a  transparent  gel.  The  process  is 
similar  to  the  electrolysis  of  salts  where  the  acid  has  a  high  molecular 
weight,  and  also  to  that  of  some  dyestuffs. 

*  R.  Zsigmondy:  Liebigs  Annalen,  301,  369  (1898). 
t  Ibid.,  370. 


154  CHEMISTRY  OF  COLLOIDS 

Protective  Effect  of  Colloidal  Stannic  Acid.  —  Contrary  to  the  be- 
havior of  silicic  acid  the  freshly  prepared  hydrosols  of  stannic  acid  have 
a  protective  effect  on  colloidal  gold,  sometimes  even  in  the  presence  of 
sodium  chloride.  If  dilute  hydrochloric  acid  is  used  instead  of  sodium 
chloride,  a  deep  red  or  purple  red  precipitate  is  obtained,  which  is 
known  as  the  gold  purple  of  Cassius. 

a  and  p  Stannic  Acid 

Two  sorts  of  stannic  acid  are  known  in  experimental,  and  also  in 
analytical  chemistry.  They  have  been  called  a  and  /?,  or  ordinary  and 
metastannic  acid.  The  first  modification  is  prepared  according  to  the 
well-known  method  of  treating  a  solution  of  stannic  chloride  with  a 
limited  amount  of  alkali;  the  second  is  made  from  the  first  by  differ- 
ent processes.  The  /3  form  is  the  more  stable  because  the  a  form  always 
turns  to  the  /?,  or  to  a  product  intermediate  between  a  and  0.  The 
latter  may  be  made  directly  by  the  action  of  nitric  acid  on  tin.  In  the 
water  content  the  two  modifications  do  not  differ  materially,  as  shown 
in  van  Bemmelen's  article.* 

In  a  moist  atmosphere  the  a  form  takes  up  more  water  than  does 
the  j8.  The  differences  of  the  chemical  properties  are  treated  in  any 
standard  work  on  analytical  chemistry.  It  seems  very  probable  that 
these  reactions  are  between  different  colloidal  modifications.  If  one 
attempts  to  get  them  in  crystalloidal  form,  which  can  be  done  by 
treatment  with  concentrated  alkalis  or  acids,  identical  solutions  are 
obtained,  e.g.,  stannates  or  SnCU,  etc.  The  hydrochloric  acid  solu- 
tion, where  the  acid  is  not  too  concentrated,  contains  beside  the  stannic 
chloride  a  considerable  amount  of  colloidal  stannic  acid,  or  colloidal 
oxychloride.  The  presence  of  this  latter  can  be  easily  demonstrated 
by  the  addition  of  red  gold  solution.  The  protective  action  is  so  great 
that  acids  will  not  precipitate  the  gold.  Stannic  chloride  solutions 
immediately  after  dilution  contain  a  considerable  quantity  of  colloid 
formed  by  hydrolysis;  in  fact  enough  to  have  a  protective  action,  f 
Only  concentrated  solutions  of  stannic  chloride  precipitate  the  gold 
and  cause  the  customary  change  of  color.  This  is  also  true  of  the  0 
form  regardless  of  whether  it  has  been  made  from  a  stannic  acid  by 
standing,  or  by  peptisation  of  the  body  made  from  nitric  acid  and  tin. 

It  is  quite  possible  that  the  differences  in  the  chemical  reactions  of 
the  two  modifications  may  be  explained  on  colloidal  grounds,  without 
the  assumption  of  isomers  or  polymers.  The  somewhat  turbid  appear- 

*  van  Bemmelen:  Ber.,  13,  1466-1469  (1880). 

t  P.  Pfeiffer:  Ber.,  38,  2466-2470  (1905);  L.  Wohler:  Koll.-Zeit.  7,  243-249 
(1910). 


COLLOIDAL  OXIDES  155 

ance,  and  the  lesser  stability  toward  electrolytes  of  metastannic  acid 
would  indicate  that  the  particles  were  larger  than  those  of  the  a  form. 
It  is  scarcely  to  be  doubted  that  the  large  number  of  modifications 
described  by  Fremy,  and  Musculus  are  likewise  colloidal  varieties  or 
mixtures  of  a  and  0  forms.  The  author  has  already  called  attention 
to  varieties  having  properties  midway  between  the  a  and  j8  forms,  and 
has  suggested  the  possible  significance.* 

The  idea  that  /3  stannic  acid  is  a  colloidal  modification  was  suggested 
by  van  Bemmelen  and  later  more  elaborately  presented  by  Mecklen- 
burg, f  The  latter  is  quite  right  in  holding  that  there  is  no  difficulty 
in  explaining  the  fact  that  the  two  modifications  under  suitable  condi- 
tions retain  their  specific  properties  after  coagulation;  that  is,  they 
may  be  again  dissolved  to  reform  a  and  /3  stannic  acid.  Doubtless  the 
coagulation  of  a  colloidal  solution  involves  the  union  of  the  particles  to 
form  larger  complexes,  and  these  complexes  are  not  homogeneous 
masses,  but  are  masses  of  particles  that  retain  their  individuality. 

As  already  pointed  out  )8  stannic  acid  contains  larger  particles  be- 
cause it  adsorbs  less  water  than  the  a  form.  Furthermore  a  stannic 
acid  is  obtained  from  crystalloidal  solutions  so  that  it  is  natural  to 
suppose  it  contains  smaller  particles.  Weber  {  has  shown  that  a  stan- 
nic acid  is  formed  at  first  from  nitric  acid  and  tin  if  the  solution  is 
sufficiently  cooled.  This  solution  is  to  be  considered,  according  to 
Mecklenburg,  as  a  stannic  acid  dissolved  in  nitric  acid.  On  boiling 
the  concentrated  solution  /?  stannic  acid  is  formed,  but  according  to 
Rose§  the  a  form  may  be  separated  out  if  the  solution  is  sufficiently 
dilute.  Another  experimental  fact,  leading  to  the  belief  that  the 
/3  form  contains  the  larger  particles,  is  the  reversibility  of  a  stannic 
acid  in  contradistinction  to  0.  The  latter  is  also  much  more  difficult 
to  return  to  the  crystalloidal  state. 

Stannic  Acid  Gel 

The  gel  of  stannic  acid  may  be  obtained  either  by  evaporating  the 
hydrosol  or  by  precipitation  with  electrolytes.  By  the  first  method 
a  transparent  jelly  is  obtained  that  becomes  glassy  on  drying  and  re- 
sembles the  gel  of  silicic  acid.  In  the  second  case  flocculent  masses 
are  the  result  of  the  precipitation. 

Peptisation.  —  Only  those  rich  in  water  may  be  easily  peptised, 
not  dried  residues.  In  the  theoretical  part  attention  was  called  to  a 

*  R.  Zsigmondy:  Liebigs  Annalen,  301,  361-387  (1898). 

t  W.  Mecklenberg:  Zeit.  f.  anorg.  Chemie,  64,  368-374  (1909).  van  Bemmelen: 
Recuil  d.  travaux  chim.  des  Pays-Bas,  7,  98  (1888). 

t  R.  Weber:  Poggendorffs  Annalen,  122,  358-371  (1864). 

§  H.  Rose:  Journ.  f.  prakt.  Chemie,  45,  76-86  (1848). 


156  CHEMISTRY  OF   COLLOIDS 

condition  of  vital  importance  for  peptisation,  viz.,  that  the  particles 
must  be  very  fine.  We  can  suppose  now  that  the  hydrogel  contains 
the  same  particles  in  aggregate  that  existed  in  the  hydrosol  and  that 
these  particles  have  the  same  individuality  in  both  conditions  except 
that  the  electric  charge  is  lacking  in  the  gel,  and  the  ultramicrons  have 
therefore  a  greater  cohesion  for  one  another.  When  alkali  is  added 
the  particles  diffuse  into  the  liquid  as  already  described  on  page  75. 
By  gradual  dehydration  the  bonds  between  the  particles  become 
stronger  and  peptisation  is  thus  rendered  more  difficult.  Finally  when 
the  dehydration  is  complete  the  particles  unite  so  that  peptisation  is  now 
impossible.  The  gradual  change  of  properties  toward  the  irreversible 
as  dehydration  progresses,  is  a  common  result  in  colloidal  chemistry. 
The  simplest  explanation  for  this  phenomenon  is  that  just  offered. 

Purple  of  Cassius 

The  purple  of  Cassius  is  that  purple  or  brown  precipitate  that  is  ob- 
tained when  a  gold  solution  is  treated  with  stannous  chloride.  The  red 
coloration  that  precedes  the  precipitation  is  a  well-known  delicate  test 
for  gold.  The  precipitate  is  used  extensively  in  china  painting  because 
of  the  beautiful  red  color.  It  was  discovered  by  Andreas  Cassius  in 
Ley  den  in  1663  and  because  of  its  importance  in  ceramics  has  been 
made  by  various  processes.  Beside  the  method  mentioned  above  the 
precipitate  may  be  obtained  by  treating  an  alloy  of  gold,  tin  and  silver 
with  nitric  acid.  The  stannous  chloride  procedure  is  as  follows.* 

200  cc.  gold  chloride  solution,  containing  3  g.  gold  as  HAuCU  per 
liter,  and  250  cc.  of  a  solution  of  stannous  chloride  (3  g.  tin  as  SnCU 
per  liter,  and  a  very  small  excess  of  hydrochloric  acid)  are  poured  into 
4  liters  of  water  and  violently  shaken.  On  standing  for  about  three 
days  the  dark  violet  red  powder  will  have  fallen  to  the  bottom  and  the 
liquid  should  be  clear  and  colorless.  The  latter  contains  neither  gold 
nor  tin.  The  precipitate  should  be  washed  by  decantation  until  there  is 
no  test  for  chlorides,  filtered  by  suction  and  again  washed.  It  is  next 
stirred  up  in  water  and  a  very  little  concentrated  ammonia  added, 
when  the  gold  should  dissolve  to  form  a  clear  solution  on  boiling. 

Richter,  Gay-Lussac  and  others  have  contended  that  the  purple  is  a 
mixture  of  gold  and  stannic  acid.  Several  writers,  among  others 
Berzelius,  have  objected  to  this  point  of  view.  Berzelius  was  aware 
of  the  existence  of  mixtures  of  gold  and  stannic  acid  but  noted  that  the 
latter  were  brick  red  and  somewhat  turbid.  Further,  these  solutions 
could  be  treated  with  aqua  regia  whereby  the  gold  and  the  stannic 
acid  could  be  separated.  Such  is  not  so  in  the  case  of  the  purple  of 
*  R.  Zsigmondy:  Liebigs  Annalen,  301,  365  (1898). 


COLLOIDAL  OXIDES  157 

Cassius.  The  solubility  in  ammonia  indicates  a  compound  of  a  chem- 
ical nature,  for  according  to  Berzelius  when  the  stannic  acid  dissolves 
in  ammonia  the  gold  should  remain  behind  if  the  mass  were  only  a 
mixture.  He  conceived  the  purple  to  be  a  chemical  combination  hav- 
ing the  composition  indicated  by  the  formula: 

Au202 .  2  Sn203 .  x  H20. 

According  to  this  point  of  view  the  solubility  in  ammonia  is  due  to  the 
formation  of  a  salt  similar  to  that  of  carminic  acid. 

Well  prepared  purple  solutions  have  a  homogeneity  scarcely  less 
than  that  of  the  crystalloidal  dyestuffs.  The  behavior  toward  mer- 
cury is  another  evidence  of  the  correctness  of  the  view  upheld  by  Ber- 
zelius. Metallic  mercury  dissolves  gold  very  easily,  while  the  gold  in 
purple  is  not  attacked  by  mercury.  The  electrolysis  of  purple  solu- 
tions resembles  closely  that  of  a  complex,  or  that  of  a  dyestuff  dis- 
solved by  an  electrolyte,  such  as  Methyl  Orange.  Just  as  in  the  case  of 
the  last  salt  dimethylamidoazobenzolsulfonic  acid  is  deposited  on  the 
anode  in  crystalline  form,  so  is  purpuric  acid  precipitated  on  the  anode 
in  form  of  a  gel. 

Nevertheless  on  the  evidence  we  must  abandon  the  view  of  Berzelius. 
It  should  be  noted  that  mere  traces  of  ammonia  will  dissolve  large 
amounts  of  purple,  three  grams  of  ammonia  suffice  f6r  1000  g.  of  dehy- 
drated purple.  From  this  it  would  follow  that  purpuric  acid  must 
have  an  enormous  molecular  weight.  Moreover  purple  will  not  pass 
through  parchment  paper  during  electrolysis  as  electrolytes  do.  Finally, 
methods  for  the  preparation  of  purple  solution  settle  the  question  for 
us.*  A  mixture  of  colloidal  gold  and  colloidal  stannic  acid  in  correct 
proportions  treated  with  dilute  acid  gives  a  precipitate  that  is  identical 
in  every  way  with  purple  solutions  made  according  to  the  previously 
mentioned  methods.  One  would  scarcely  expect  the  inactive  gold  to 
unite  with  the  comparatively  inactive  stannic  acid.  In  fact  if  submi- 
croscopic  gold  particles  are  chosen  for  the  preparation  of  synthetical 
purple  solutions  the  particles  may  be  seen  under  the  ultramicroscope  just 
as  easily  after  the  reaction  as  before  the  solutions  were  mixed.  The  ab- 
sorption spectra  of  ordinary  and  synthetic  purple  solutions  are  iden- 
tical. All  this  goes  to  show  that  the  gold  has  not  entered  into 
chemical  combination  although  the  reactions  have  so  materially  changed. 
The  analysis  of  the  above  arguments  is  important  in  colloidal  chemistry 
because  certain  authors  have  used  these  same  arguments  in  the  case  of 
many  other  colloids.f 

*  R.  Zsigmondy:  Liebigs  Annalen,  301,  375  (1898). 

t  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  56  (1905). 


158  CHEMISTRY  OF  COLLOIDS 

The  peptisation  of  the  purple  of  Cassius  is  also  important  for  the 
explanation  of  this  phenomenon.  As  long  as  the  point  of  view  of  Ber- 
zelius  was  held  peptisation  could  be  attributed  to  the  formation  of  a 
salt.  Since  this  is  no  longer  tenable  we  must  accept  some  such  ex- 
planation as  has  been  given  in  the  theoretical  part  of  this  book.  See 
Chapter  IV. 

Substances  Analogous  to  the  Purple  of  Cassius 

Allied  to  the  purple  of  Cassius  are  those  substances  composed  of 
colloidal  metals  and  colloidal  stannic  acid.  A  good  instance  of  this 
is  silver  purple.  Silver  nitrate  and  stannous  nitrate  form  a  blood-red 
solution  that  gradually  turns  brown  and  from  which  a  reddish  brown 
precipitate  falls  out.  Ditte  *  mistook  this  for  silver  stannate,  but 
L.  Wohler  f  has  demonstrated  that  the  substance  is  analogous  to  gold 
purple.  Lottermoser  {  has  succeeded  in  preparing  another  silver  pur- 
ple synthetically  by  mixing  colloidal  silver  and  colloidal  stannic  acid. 
Acids  throw  out  a  dark  violet  precipitate  that  dissolves  in  alkali  with 
a  deep  brown  color.  The  same  product  may  be  obtained  by  mixing 
solutions  of  silver  nitrate,  stannic  chloride  and  ammonium  citrate. 

According  to  L.  Wohler  §  an  analogous  platinum  combination  may  be 
made  from  stannous  chloride  and  platinum  chloride.  A  blood-red 
color  ensues  that  was  formerly  attributed  to  the  presence  of  platinous 
chloride,  but  has  since  been  shown  to  be  finely  divided  metal.  At 
sufficiently  great  dilution  a  brown  precipitate  falls  out.  This  may  be 
washed  free  from  chlorides  and  then  has  the  following  composition: 

PtSn6Oi2. 

Other  substances  have  been  made  by  the  same  process  and  have  the 
composition: 

Pt(Sn02)8  and  Pt(Sn02)5. 

These  products  have  all  the  properties  of  colloids.  They  show  the 
effects  of  ageing,  when  freshly  prepared  are  soluble  in  ammonia  or  dilute 
hydrochloric  acid,  and  after  drying  are  scarcely  at  all  soluble  in  concen- 
trated hydrochloric  acid.  The  red  substance  does  not  diffuse  through 
membranes. 

Another  peculiar  reaction  might  be  noted.  If  made  in  strong  hydro- 
chloric acid  solution  the  red  platinum  purple  is  soluble  in  ether  and  in 
ethyl  acetic  ether,  so  that  it  may  be  separated  from  the  water  by  shak- 

*  A.  Ditte:  Annales  de  Chim.  et  de  Phys.  (5),  27,  145-182  (1882);  J.  B.,  343, 
1301  (1882). 

t  L.  Wohler:  Koll.-Zeit.,  7,  248  (1910). 

t  A.  Lottermoser:  Anorganische  Kolloide,  53.     Stuttgart  (1901). 

§  L.  Wohler:  Koll.-Zeit.  2,  Suppl.  1,  111,  53  (1907). 


COLLOIDAL  OXIDES  159 

ing  with  these  solvents.  The  ether  solution  leaves  a  residue  soluble  in 
water.  It  would  seem  that  a  stannic  oxychloride  plays  the  role  of  a 
protective  colloid  in  this  case,  and  that  the  ether  modification  does  not 
form  a  complex  crystalloid. 

Metastannic  Acid  Containing  Iron 

In  the  purple  of  Cassius  we  have  an  amalgamation  of  the  properties 
of  both  components.  The  colloidal  gold  is  responsible  for  the  color 
and  a  part  of  the  optical  properties,  while  most  of  the  reactions  are 
those  of  stannic  acid.  Only  in  case  the  gold  is  the  larger  portion  of 
the  purple  does  coagulation  by  electrolytes  come  into  consideration. 
Toward  other  colloids  both  components  exert  the  individual  reactions 
of  each.  An  interesting  case  has  been  observed  by  Lepez  and  Storch.* 
When  tin  is  dissolved  in  hot  nitric  acid  metastannic  acid  is  formed.  If, 
however,  ferric  nitrate  has  been  added  to  the  nitric  acid  metastannic 
acid  containing  iron  is  formed,  and  this  exhibits  entirely  different 
reactions  from  ordinary  metastannic  acid.  As  is  well  known,  meta- 
stannic acid  becomes  insoluble  if  water  is  added  to  the  mixture.  If, 
however,  the  colloidal  combination  contains  one  atomic  weight  of  iron  to 
one  of  tin  the  mass  will  dissolve  when  it  is  diluted  with  water.  When 
the  stannic  acid  is  present  in  larger  proportions  than  the  above  the 
mass  loses  its  solubility  in  water,  but  may  be  dissolved  in  concentrated 
hydrochloric  acid. 

Not  only  are  the  reactions  of  metastannic  acid  partially  hidden  by 
the  mixture,  but  also  those  of  iron  oxide.  If  sufficient  stannic  acid  is 
in  the  precipitate,  ammonia  will  cause  peptisation.  This  is  contrary 
to  the  behavior  of  iron  oxide,  of  course,  because  the  latter  is  thrown 
out  of  solution  by  ammonia. 

The  property  that  stannic  acid  has  of  preventing  the  precipitation 
of  many  oxides  such  as  those  of  bismuth,  copper,  lead,  etc.,  and  like- 
wise the  precipitation  of  phosphoric  acid,  is  well  known  in  analytical 
chemistry. 

C.  Colloidal  Titanic  Acid,  Colloidal  Zirconium  Oxide  and  Colloidal 

Thorium  Oxide 

Colloidal  titanic  acid  can  be  prepared  by  Graham's  f  method,  viz., 
the  addition  of  ammonia  to  salts  of  titanium  in  solution.  The  precipi- 
tate is  peptised  by  nitric  acid  and  then  dialyzed.  The  dialysis  of  the 
nitrate,  a  method  followed  by  Biltz,J  is  applicable  to  the  preparation 

*  C.  Lepe"z  und  L.  Storch:  Monatshefte  f.  Chemie,  10,  283-294  (1889). 
.  f  Th.  Graham:  Annalen  d.  Chemie  u.  Pharmazie,  135,  65-79  (1865). 
t  W.  Biltz:  Ber.,  35,  4431-4438  (1902). 


160  CHEMISTRY  OF  COLLOIDS 

of  colloidal  zirconium  and  thorium  oxides,  and  indeed  to  all  colloidal 
oxides  of  trivalent  or  quadrivalent  metals.  Nitrates  of  the  bivalent 
metals  are  not  sufficiently  hydrolyzed  to  make  this  method  possible 
in  their  case. 

Colloidal  Zirconium  Oxide.  —  As  already  stated  colloidal  zirconium 
oxide  is  made  by  dialyzing  the  nitrate.  Solutions  having  a  concentra- 
tion of  about  17  per  cent  are  dialyzed  for  5  days.  Hydrosols  thus 
obtained  contain  particles  that  are  charged  positively,  and  give  a  pre- 
cipitate with  negative  colloids.  With  gold  in  suitable  proportions,  for 
instance,  a  red  precipitate  comes  down,  which  is  analogous  to  the 
purple  of  Cassius.  Biltz  *  states  that  the  solution  has  decided  pro- 
tective properties.  When  HC1  is  employed  the  gold  number  has  been 
found  by  Biltz,*  and  also  by  Behref  to  be  0.05. 

Zirconium  oxy chloride  is  hydrolyzed  according  to  Ruer,t  and  the 
reaction  may  be  hurried  by  a  rise  of  temperature.  The  dialysis  of  such 
solutions  gives  a  hydrosol  of  zirconium  oxide.  A  further  change  in 
the  solution  results  in  the  formation  of  metazirconic  acid,  and  the 
reactions  are  accordingly  different  from  the  original  hydrosol. 

Both  the  oxychloride  and  the  nitrate  give  a  precipitate  with  oxalic 
acid,  which  precipitate  is  soluble  in  an  excess  of  the  reagent.  This 
reaction  does  not  take  place  in  the  presence  of  sulfates  because  of  the 
formation  of  a  complex  in  which  the  zirconium  is  in  the  anion.  The 
probable  formula  is  K2[ZrO(S04)2].  When,  however,  the  solution  is 
aged  and  hydrolysis  has  sufficiently  progressed  so  that  metazirconic  acid 
has  been  formed,  the  presence  of  sulfates  no  longer  serve  to  cover  up 
the  action  of  oxalic  acid.  In  fact  old  solutions  if  warmed  will  give 
precipitates  with  sulfates  that  are  soluble  in  an  excess  of  the  reagent. 
Freshly  prepared  zirconium  oxychloride  or  nitrate  solutions,  on  the 
other  hand,  give  no  precipitate  with  sulfate.  These  changes  that 
take  place  on  ageing  are  doubtless  responsible  for  many  discrepancies 
in  the  work  of  different  investigators. 

Colloidal  Metazirconic  Acid.  —  On  boiling,  the  hydrosol  of  zirconic 
acid  changes  to  metazirconic  acid.  For  the  preparation  of  the  met  a 
acid,  metazirconium  chloride  may  be  employed.  This  latter  substance 
is  made  by  repeated  evaporation  of  the  oxychloride.  By  dialyzing 
the  solution  a  milky  liquid  is  obtained  that  closely  resembles  meta- 
stannic  acid.  Concentrated  HC1  produces  a  precipitate  that  is  soluble 
in  water.  Concentrated  solutions  of  alkali  chloride  behave  similarly. 
The  hydrosol  is  completely  precipitated  by  sulfuric  acid  and  the  pre- 

*  W.  Biltz:  Ber.,  35,  4431-4438  (1902). 

t  P.  Behre:  Inaug.-Piss.  Gottingen  (1908). 

j  R.  Ruer:  Zeit.  f.  anorg.  Chemie,  43,  282-303  (1905). 


COLLOIDAL  OXIDES  161 

cipitate  is  not  at  all  soluble  in  an  excess  of  the  acid.  The  same  is  true  of 
the  action  of  sodium  sulfate.  The  dehydrated  residue  of  this  hydrosol 
is  amorphous  and  opaque  in  contradistinction  to  the  transparent  resi- 
due from  ordinary  zirconium  oxide  hydrosol. 

The  hydrogel  of  metazirconic  acid  is  obtained  by  precipitating  the 
metachloride  with  ammonia.  It  is  not  so  voluminous  as  the  ordinary 
variety  and  behaves  differently  on  being  heated.  The  latter  glows  on 
being  heated  while  the  former  does  not.  The  heat  of  reaction  during 
the  change  from  the  ordinary  form  to  the  more  stable  is  9.2  cal. 

Colloidal  Thorium  Oxide.  —  Colloidal  thorium  oxide  obtained  by  dia- 
lyzing  the  nitrate  is  clear  and  neutral,  but  contains  traces  of  nitrate.* 
On  evaporation  there  remains  a  rubber-like  lustrous  mass  that  is  not 
soluble  in  water.  The  hydrosol  is  very  stable  toward  electrolytes; 
dilute  HC1  and  even  normal  sodium  chloride  solution  causes  no  reaction. 
More  concentrated  solutions  of  sodium  chloride,  or  30  per  cent  ammo- 
nium sulfate  cause  a  precipitate.  This  stability  is  diametrically  opposed 
to  the  great  susceptibility  of  colloidal  cerium  or  zirconium  oxides.  A 
peculiar  preparation  for  thorium  oxide  is  given  by  Cleve.f  Thorium 
oxide  evaporated  down  with  HC1  is  not  changed  in  outward  appear- 
ance, but  is,  nevertheless,  soluble  in  water  and  forms  a  turbid  liquid. 
Szillard  J  has  made  a  stable  hydrosol  of  thorium  oxide  by  peptising  a 
well  washed  sample  of  thorium  hydrate  with  nitric  acid.  A.  Miiller§ 
obtained  a  reversible  hydrosol  by  peptising  the  gel  of  thorium  hydroxide 
with  HC1  at  boiling  temperature.  The  well  washed  gel  is  thrown  out  in 
boiling  water  and  peptised  by  N/20  HC1.  On  evaporating  this  hydrosol 
a  rubber-like  residue  remains  that  distends  in  water  and  forms  a  viscous 
liquid.  This  "  mineralische  Gummi "  dries  on  paper  to  form  a  lustrous 
layer  that  will  stick  pieces  of  paper  together  as  gum  arabic  does.  This 
hydrosol  reacts  as  a  semicolloid,  and  its  properties  are  probably  due  to 
excess  of  HC1  used  in  peptisation  which  has  not  been  removed  by  dialy- 
sis. As  in  the  case  of  all  positively  charged  colloids  this  hydrosol  will 
precipitate  all  negatively  charged  colloids  including  colloidal  gold. 

Peptoids 

Since  the  time  of  Graham  1f  it  is  well  known  that  a  certain  analogy 
exists  between  peptone  formation  from  egg  albumin  and  inorganic 
peptisation.  Szillard  ||  has  endeavored  to  elaborate  on  the  analogy. 

*  I.e. 

f  P.  T.  Cleve:  Bull.  Soc.  Chim.  (2),  21, 115-123.    Paris  (1874);  J.  B.,  251  (1874). 

t  B.  Szillard:  Journ.  de  chim.  phys.,  6,  48&-404  (1907). 

§  A.  Miiller:  Koll.-Zeit.  2,  Suppl.  1,  VI-VIII  (1907). 

i  Th.  Graham:  Phil.  Transact.,  183  (1861);  Liebigs  Annalen,  121,  45  (1862). 

||  B.  Szillard:  Journ.  de  chim.  phys.,  6,  495,  636-646  (1907). 


162  CHEMISTRY  OF  COLLOIDS 

Coagulated  egg  albumin  digested  with  pepsine  is  changed  into  pep- 
tones and  albumoses  that  have  a  greater  degree  of  homogeneity  than 
the  original  albumin.  Likewise  the  hydrogels  of  different  inorganic 
colloids  may  peptise  with  HC1  or  alkalis,  and  even  with  salts  of  the 
heavy  metals.  As  an  example  of  this  may  be  cited  Graham's  iron 
oxide  hydrosol.  Szillard  has  shown  that  the  peptisation  with  salts  of 
the  heavy  metals  is  general,  and  that  a  whole  series  of  new,  mixed 
heterogeneous  colloids  may  be  obtained  by  peptising  the  hydrogel  of 
one  metal  oxide  with  the  salt  of  another  having  a  high  valence.  These 
new  hydrosols  have  been  called  by  Szillard,  peptoids.  Two  examples 
of  the  series  have  been  chosen  for  purposes  of  illustration. 

1.  Uranium  hydrate  thorium  peptoid. 

2.  Thorium  hydrate  uranyl  peptoid. 

The  first  is  made  by  peptising  uranyl  hydrate  with  thorium  nitrate; 
the  second  by  peptising  thorium  hydrate  with  uranyl  nitrate.  The 
first  is  a  delicate  yellowish  green,  while  the  second  is  a  dark  reddish 
yellow.  With  regard  to  the  preparation  it  should  be  noted  that  the 
hydrate  must  be  well  washed,  whereby  a  suspension  is  formed.  The 
solution  of  the  nitrate  is  raised  to  the  boiling  point  and  gradually 
added  to  the  suspension. 

The  reaction  itself  may  be  explained  on  the  following  grounds.  The 
first  portions  of  the  hydrogel  are  either  peptised  by  the  cathion  of  the 
salt,  or  dissolved  by  the  nitrate  to  form  a  basic  salt.  These  combina- 
tions next  form  adsorption  complexes  with  more  oxide,  the  anions 
dissociate  off  and  the  complex  is  therefore  positively  charged.  That 
different  products  are  formed  is  evidenced  by  the  variety  of  colors  in 
the  hydrosols.  Coagulated  egg  albumin  may  also  be.  peptised  by  the 
salts  of  the  heavy  metals. 

D.  Colloidal  Iron  Oxide 

Graham's  *  method  for  preparing  colloidal  iron  oxide  is  to  dissolve 
iron  hydroxide  in  a  solution  of  ferric  chloride  and  then  dialyze.  Krecke  f 
dialyzed  a  dilute  solution  of  ferric  chloride,  while  Biltz  {  employed  the 
nitrate.  Colloidal  iron  oxide  having  somewhat  different  properties  has 
been  made  by  Pe*an  de  St.  Gilles  t  by  continued  boiling  of  a  solution  of 
ferric  acetate.  Graham  has  called  these  solutions  colloidal  metairon 
oxide. 

The  preparation,  according  to  the  method  of  Graham,  is  colored  deep 
brown,  is  comparatively  free  from  electrolytes,  but  always  contains 

*  F.  Krecke:  Journ.  f.  prakt.  Chemie  (2),  3,  286-306  (1871). 

t  W.  Biltz:  Ber.,  35,  4431-4438  (1902). 

J  Pean  de  St.  Gilles:  Compt.  rend.,  40,  568-571,  1243-1247  (1855). 


COLLOIDAL  OXIDES  163 

traces  of  chlorides  that  cannot  be  completely  removed.  Most  electro- 
lytes cause  precipitation,  but  dilute  acids  merely  render  the  hydrosol 
more  stable.  Hydrosols  with  properties  similar  to  those  of  Graham 
and  having  a  concentration  of  about  5  per  cent  may  be  had  on  the 
market.  Merck  handles  a  10  per  cent  preparation  that  contains  rela- 
tively more  chloride  than  does  the  5  per  cent  preparation. 

The  brown  color  of  ferric  acetate  becomes  gradually  brick  red  on 
boiling  and  the  peculiar  taste  of  ferric  acetate  becomes  more  like  that 
of  acetic  acid.  Hydrosols  prepared  by  Graham's  method  contain 
smaller  particles  than  those  of  Pean  de  St.  Gilles.  In  the  hydrosols  on 
the  market  one  is  able  to  discern,  beside  the  individual  particles,  homo- 
geneous light  cones.  On  diluting,  these  cones  gradually  fade  away 
without  being  resolvable.  Hydrosols  from  ferric  acetate  give  much 
more  intense  light  cones,  and  contain,  therefore,  much  larger  particles. 
Coehn  *  has  shown  that  the  iron  oxide  hydrosols  mentioned  above  are 
all  charged  positively;  but  recently  Fischer  f  has  prepared  a  negatively 
charged  one. 

Reactions  of  Colloidal  Iron  Oxide.  —  Colloidal  iron  oxides  obtained 
from  the  acetate  are  turbid  in  reflected  light  and  clear  in  transmitted. 
Sulfuric  acid  or  its  salts  coagulate  the  hydrosol  immediately,  and  the 
precipitate  is  not  soluble  in  concentrated  acid  in  contradistinction  to 
the  precipitate  from  Graham's  solutions.  When  the  hydrosol  is  poured 
into  concentrated  acid  a  precipitate  is  formed  that  is  again  dissolved  by 
water.  There  exist  several  analogies  between  the  hydrosol  of  meta- 
stannic  acid  and  those  of  these  ferric  acetate  hydrosols;  this  accounts 
for  the  name,  metairon  oxide.  The  hydrosol  deserves  more  elaborate 
investigation;  the  apparent  isomerism  of  the  two  modifications  is  prob- 
ably due  to  the  difference  in  the  size  of  the  particles. 

Graham's  iron  oxide  differs  from  the  meta  variety  in  that  the  former 
is  more  stable  and  its  precipitate  is  more  soluble  in  acids.  Colloidal  iron 
oxide  on  the  market  has  properties  that  lie  between  these  two  extremes. 
It  is  precipitated  by  moderately  concentrated  HC1  but  the  solid  gradu- 
ally goes  into  solution  again  to  form  a  chloride.  This  colloid  there- 
fore bears  the  same  relation  to  Graham's  and  metairon  oxide  that 
colloidal  stannic  oxide  prepared  by  the  author's  method  bears  to  the 
a  and  /3  varieties. 

Reference  may  be  made  to  a  remark  of  Graham  \  that  is  pertinent  to 
all  colloids.  Where  it  is  solely  a  case  of  some  interchange  or  reaction 
between  the  particles  the  process  may  take  place  quickly,  such  as  the 

*  A.  Coehn:  Zeit.  f.  Elektrochemie,  4,  63-67  (1897-98). 

t  H.  W.  Fischer  und  E.  Kusnitzky:  Biochem.  Zeit.,  27,  311-325  (1910). 

%  Th.  Graham:  Liebigs  Annalen,  121,  70-71  (1862). 


164  CHEMISTRY  OF  COLLOIDS 

precipitation  of  iron  oxide  by  electrolytes.  On  the  other  hand,  where 
true  chemical  reactions  are  involved,  colloids  demand  an  appreciable 
(sometimes  a  long)  time  for  their  completion.  This  is  exemplified  by 
the  slow  dissolution  of  the  precipitated  iron  oxide  in  the  acidified 
liquid. 

Other  Reactions  of  Colloidal  Iron  Oxide.  —  The  want  of  the  char- 
acteristic iron  taste,  or  ink  taste,  is  entirely  lacking,  due  doubtless  to 
the  scarcity  of  the  ferric  ion  in  the  colloidal  solution.  The  hydrosol 
gives  only  a  rough  feel  on  the  tongue.  Another  evidence  *  that  the 
ferric  ion  is  present  only  in  extremely  small  amount  in  well  dialyzed 
hydrosols  is  the  fact  that  potassium  ferrocyanide  gives  no  Prussian  blue, 
although  this  is  a  very  delicate  test  for  ferric  ion.  Silver  nitrate  fails 
to  show  the  presence  of  the  chloride  ion.  This  is  of  special  interest 
because  Ruer,f  and  Hantzsch  and  Desch  {  all  have  demonstrated  that 
some  chloride  is  present.  The  two  latter  authors  attribute  the  failure 
of  silver  nitrate  to  show  a  chloride  reaction  to  the  formation  of  a  chlo- 
ride complex.  Ruer,  however,  ascribes  the  phenomenon  to  a  protec- 
tive action  of  the  colloidal  iron  oxide.  That  is  to  say,  the  particles  of 
silver  chloride  do  not  become  large  enough  to  cause  a  turbidity.  For 
instance,  a  solution  containing  0.752  g.  Fe203,  0.220  g.  Cl  in  100  cc.  gives 
only  a  faint  opalescence  with  silver  nitrate.  If  the  solution  is  boiled 
with  nitric  acid  so  that  the  iron  is  dissolved,  silver  chloride  is  at  once 
apparent.  Ruer  proved  that  the  hydrosol  actually  contains  the  chloride 
ion  by  dialyzing  without  changing  the  water.  The  outer  vessel  con- 
tained 0.0056  per  cent  Fe203  and  0.0760  per  cent  Cl;  while  the  inner 
contained  0.7032  Fe203  and  0.1388  per  cent  Cl.  The  outer  liquid  gave 
a  good  test  for  chloride  ion  while  the  inner  did  not.  As  there  must  be 
an  equilibrium  between  the  chloride  ion  in  the  two  liquids  it  is  clear 
that  the  non-appearance  of  silver  chloride  in  the  inner  vessel  must  be 
due  to  some  protective  action. 

A  series  of  interesting  and  theoretically  important  researches  have 
been  carried  out  by  Duclaux  §  during  his  work  on  Hardy's  precipita- 
tion law.  He  experimented  on  solutions  of  colloidal  copper,  ferrocya- 
nide and  colloidal  iron  oxide.  Different  sorts  of  colloidal  iron  oxide 
were  treated  with  a  series  of  electrolytes,  with  the  result  that  10  cc.  of 
the  hydrosol  were  precipitated  by  almost  equivalent  quantities  of 
anions  regardless  of  the  valency.  For  instance,  10  cc.  of  a  hydrosol 
having  0.0203  gram  atoms  of  iron  in  a  liter,  0.00166  gram  atoms  of  Cl 

*  H.  W.  Fischer  und  E.  Kusnitzky:  Biochem.  Zeit.,  27,  311-325  (1910). 
t  R.  Ruer:  Zeit.  f.  anorg.  Chemie,  43,  85-93  (1905). 
j  A.  Hantzsch  und  C.  Desch:  Liebigs  Annalen,  323,  28-31  (1902). 
§  J.  Duclaux:  Journ.  de  Chim.  Phys.,  5,  29-56  (1907). 


COLLOIDAL  OXIDES 


165 


in  a  liter  required  for  coagulation  the  gram  equivalents  of  various 

anions  given  in  Table  24. 

TABLE  24 


17 

10-6 

Gram  equivalents 

Sp4= 

16.5 

10-6 

citrate  ion 

15.2 

io-« 

CrOr 

17 

10-6 

CO3= 

19 

io-« 

P04= 

16.1 

10-6 

OH~ 

13 

10-* 

FeCy6-z 

1880 

io-« 

N03~ 

It  was  further  shown  that  the  amount  of  sulfate  or  hydroxide  ion 
necessary  for  precipitation  was  almost  equivalent  to  the  chloride  con- 
tent of  the  hydrosol.  The  relations  for  a  solution  of  iron  oxide  con- 
taining 203  •  10~6  gram  atoms  of  iron  are  given  in  Table  25. 

TABLE  25 


Gram  equivalent. 

SO4- 

OH- 

NOs- 

17  •  10-6 

17-  10-« 

16  -  10-6 

1880  •  10-6 

8-10-6 

6.8-  10-« 

6.6-  10-6 

440  -  10-6 

4.1  •  10-6 

4  •  lO"6 

.    3.6-10-6 

70  •  10-6 

2.8-10-6 

2-10-« 

2.2-10-« 

36  •  10-6 

There  seems  to  be  no  equivalent  relation  for  the  nitrate  ion.    The  same 
want  of  relation  also  exists  for  sodium  chloride  as  may  be  seen  in  Table  26. 

TABLE  26 
IRON  OXIDE  HYDROSOL 


Precipitated  by  gram  equivalents. 

With  gram  atom 

Cl. 

804= 

NaCl 

'     11 

13 

2000 

7.2 

7.2 

170 

4.8 

3.4 

75 

1 

0.9 

6 

With  the  exceptions  of  the  nitrate  ion  and  sodium  chloride,  equiva- 
lent quantities  of  the  anions  cause  the  same  effect.  That  the  quanti- 
ties are  not  exact  is  to  be  expected  when  one  considers  that  the  ex- 
perimental error  is  large  owing  to  the  fact  that  precipitation  depends 
somewhat  on  the  manner  of  adding  the  electrolyte. 

The  Schulze-Hardy  precipitation  law  does  not  hold  for  the  nitrate 
ion  nor  for  sodium  chloride.  In  explanation  the  assumption  may  be 


166  CHEMISTRY  OF  COLLOIDS 

made  that  colloidal  iron  oxide  is  a  solution  of  an  oxychloride  having  a 
high  molecular  weight,  the  cathion  of  which  unites  with  the  anion  of  the 
electrolyte  to  form  an  insoluble  precipitate.  Or  one  may  assume  that 
the  iron  ultramicrons  have  adsorbed  the  cathion  of  the  oxychloride  as 
represented  on  page  79.  The  latter  assumption  is  the  more  general 
and  will  be  dealt  with  at  length. 

The  adsorbed  cathion,  the  nitrate  and  chloride  of  which  are  soluble, 
gives  the  positive  charge  to  the  ultramicrons  and  is  responsible  for  the 
stability  of  the  hydrosol.  In  order  to  cause  the  precipitation  of  the 
insoluble  union  (Fe-complex-cathion)m  (anion) n,  it  will  be  necessary 
to  add  enough  of  the  reagent  to  correspond  to  the  chloride  ion  in  solu- 
tion, hence  the  equivalence  between  the  precipitating  anion  and  the 
chloride  content  of  the  colloid.  To  make  this  clear  let  us  assume  that 
the  absorbed  cathion  has  the  formula,  Fe202++  and  the  undissociated 
salt  the  formula,  Fe202Cl2.  The  following  formulas  would  therefore 
represent  the  adsorption  of  the  cathion  by  the  ultramicrons  and  finally 
the  neutralization  of  the  complex  by  the  anion  of  the  addition  agent. 

|Fe2O3|  +  Fe202++  =  |Fe2O3|  •  Fe2O2++ 
Fe202++  +  2  OH~ ->  j  [Fe^A]  Fe202  (OH)2 

The  hydroxide  ion  may  be  replaced  by  SQ±~  ~,  Cr04~  ~~,  etc. 

According  to  the  assumption  the  nitrate  and  the  chloride  are  much 
more  soluble  so  that  more  of  these  precipitating  agents  must  be  added 
to  cause  coagulation. 

Conductivity  of  Colloidal  Iron  Oxide 

Malfitano*  has  shown  that  solutions  of  potassium  or  iron  chloride 
will  pass  through  collodion  membranes  without  causing  any  change  in 
conductivity.  Duclauxt  has  investigated  the  conductivity  of  colloidal 
solutions  making  use  of  collodion  membranes.  He  has  shown  that  col- 
loidal iron  oxide  made  by  Graham's  method,  and  containing  0.032  gram 
atoms  of  iron  per  liter,  gives  a  colorless  filtrate  if  filtered  through  a 
collodion  membrane. 

The  original  solution  had  the  conductivity 113  •  10~6 

The  filtrate  had  the  conductivity 82  •  10"6 

The  residue,  TV  of  the  original,  had  the  conductivity. .     280  •  10~6 
The  increase  in  the  "micellen  "  has  increased  the  conductivity. 

That  the  intermicellular  liquid  is  not  changed  by  passing  through  the 
membrane  is  shown  by  the  following  table. 

*  G.  Malfitano:  Compt.  rend.,  139,  1221  (1904). 

t  J.  Duclaux:  Compt.  rend.,  140,  1468-1470,  1544-1547  (1905);  Koll.-Zeit.,  3, 
126-134  (1908) 


COLLOIDAL  OXIDES 
TABLE  27 


167 


Time  of  measurement. 

Conductivity  of  the 
filtrate  in  arbitrary 
units. 

At  the  beginning 
Concentration  increased    4  fold 
6    " 
24    " 

100 
99 
101 
106 

Not  until  the  end  is  there  any  increase  in  the  conductivity.  This 
result  is  corroborated  by  some  researches  in  the  author's  laboratory  by 
Dr.  Bachmann.  From  this  it  is  clear  that  the  adsorption  of  the  elec- 
trolyte by  the  membrane  is  negligible,  a  result  that  is  important  in 
both  filtration  and  in  measurements  on  osmotic  pressure.  It  renders 
an  objection  raised  by  Lottermoser  invalid.*  Recently  Lottermoser 
has  become  convinced  that  the  filtration  method  of  studying  adsorp- 
tion within  a  liquid  is  suitable,  f  An  article  by  Wo.  Ostwald  t  also 
corroborates  this. 

Osmotic  Pressure.  —  Duclaux  §  has  shown  that  colloidal  iron  oxide 
exhibits  an  osmotic  pressure  against  its  filtrate,  that  it  increases  with 
the  concentration  but  is  not  proportional  to  the  latter.  When  the  con- 
centration increased  from  1  to  18  the  pressure  rose  in  the  ratio  of  1  to  80. 
The  cause  for  this  want  of  proportion  is  not  yet  very  well  understood, 
but  Duclaux  has  offered  a  somewhat  plausible  explanation.^  He  has 
also  shown  that  the  osmotic  pressure  of  iron  oxide  hydrosol  sinks  with 
a  rise  of  temperature.  The  author  carried  on  an  experiment  for  over 
.one  and  one-half  years  and  found  at  regular  intervals  that  a  rise  in 
temperature  to  50°  C.  caused  a  fall  in  the  pressure  of  about  15  mm. 
As  the  temperature  was  lowered  the  column  gradually  went  up  again. 
There  is  a  gradual  decrease  in  the  osmotic  pressure  that  is  partly 
accounted  for  by  the  changes  that  go  on  in  the  colloid,  viz.,  the  forma- 
tion of  larger  particles  from  the  smaller.  Submicrons  could  be  easily 
'seen  toward  the  end  of  the  experiment. 

Magnetic-optical  Investigations 

Form  of  the  Ultramicrons.  —  Majorana  ||  has  observed  that  col- 
loidal iron  oxide  on  a  magnetic  field  exhibits  the  properties  of  uni- 

*  A.  Lottermoser:  Zeit.  f.  phys.  Chemie,  60,  451-463  (1907). 

t  A.  Lottermoser  und  P.  Maffia:  Ber.,  43,  3613-3618  (1910). 

j  Wo.  Ostwald:  van  Bemmelen-Gedenkboek,  267-274  (1910). 

§  J.  Duclaux:  Compt.  rend.,  140,  1544  (1905). 

f  J.  Duclaux:  Journ.  de  Chim.  Phys.,  7,  405-446  (1909). 

||  Qu.  Majorana:  Rendic.  R.  Accad.  Lincei,  11,  1,  374,  463,  531;  11,  90,  139 
(1902). 


168  CHEMISTRY  OF  COLLOIDS 

axial  crystals.  The  hydrosol  of  iron  oxide  in  the  field  of  a  powerful 
electromagnet,  and  traversed  by  a  light  ray  at  right  angles  to  the  lines 
of  force,  exhibits  double  refraction  sometimes  as  great  as  that  of  quartz. 
The  double  refraction  disappears  again  when  the  magnet  is  switched 
out  of  the  circuit.  The  amount  of  the  double  refraction  depends  upon 
the  age  of  the  hydrosol. 

Schmauss  *  called  attention  to  the  fact  that  this  was  a  general  prop- 
erty of  colloids,  and  assumed  that  the  magnet  caused  an  orientation 
of  the  particles.  Cotton  and  Mouton  f  with  the  aid  of  the  ultrami- 
croscope  corroborated  the  observations  of  Schmauss,  accepted  his 
theory  and  extended  its  application  materially.  They  found  that  ultra- 
filtration  through  collodion  membranes  increased  the  double  refrac- 
tion of  the  colloid,  while  the  filtrate  remained  inactive.  It  is  found 
that  coagulation  under  ordinary  circumstances  gives  an  inactive  gel, 
but  that  if  the  coagulation  takes  place  in  the  magnetic  field  the  gel  ex- 
hibits a  permanent  double  refraction  even  after  the  removal  of  the 
magnet.  Finally,  the  turbidity  and  the  double  refraction  become 
greater  with  the  age  of  the  hydrosol;  this  change. may  be  hastened  by 
raising  the  temperature.  On  heating  four  hours  at  100°  C.  the  double 
refraction  increases  fortyfold.  The  turbidity  and  the  viscosity  increase 
during  the  warming,  therefore  the  greater  the  particles  the  greater  the 
double  refraction.  Some  hydrosols  prepared  by  electrical  colloidation 
contained  particles  that  could  be  seen  under  an  ordinary  microscope. 
These  particles  were  long  and  became  orientated  in  the  magnetic 
field,  and  simultaneously  the  hydrosol  exhibited  double  refraction.  It 
seems  plausible  therefore,  that  this  phenomenon  is  due  to  the  orienta- 
tion of  the  ultramicrons  that  are  lamellar  or  rod-shaped 4  Cotton 
and  Mouton  have  demonstrated  that  the  rods  or  flakes  do  not  form 
threads. 

It  is  now  quite  clear  why  the  larger  particles  cause  a  greater  effect 
than  the  smaller.  The  brownian  movement  tends  to  disturb  the  ori- 
entation and  the  smaller  ultramicrons  having  a  much  greater  rate  of 
motion  than  the  larger  do  not  become  orientated  sufficiently  to  cause 
pronounced  double  refraction.  From  the  researches  of  Cotton  and 
Mouton  the  question  whether  the  position  of  the  particles  alone,  or 
whether  the  particles  themselves,  cause  double  refraction,  must  be 
answered  in  favor  of  the  latter  assumption. 

*  A.  Schmauss:  Drudes  Annalen  d.  Phys.  (4),  12,  186-195  (1903). 

t  A.  Cotton  et  H.  Mouton:  Compt.  rend.,  141,  317,  349  (1905);  Soc.  fr.  de  phys., 
17,  Nov.  (1905).  Les  ultramicroscopes,  etc.,  Chap.  VIII.  Paris  (1906). 

t  O.  Wiener:  Physikal.  Zeit.,  5,  332-338  (1904).  Lord  Rayleigh:  Philos. 
Magaz.  (6),  34,  481-502  (1892).  Kerr:  Report  of  Brit.  Assoc.  Glasgow,  568  (1901). 
F.  Braun:  Physikal.  Zeit.,  6,  199-203  (1904). 


COLLOIDAL  OXIDES 


169 


The  researches  of  Cotton  and  Mouton  are  of  great  importance  for 
the  theory  of  colloidal  chemistry.  We  have  here  for  the  first  time 
direct  evidence  that  a  colloid  closely  resembling  the  lyophiles  contains 
particles  the  form  of  which  is  not  that  of  a  sphere,  but  rather  that 
of  tiny  crystals.  Nageli  *  has  explained  the  optical  and  many  other 
properties  of  colloids  on  the  grounds  that  they  consist  of  anisotropic 
ultramicrons  having  a  great  resemblance  to  crystals. 

Iron  Oxide  Hydrogels.  —  Van  Bemmelen  f  has  investigated  the  dehy- 
dration of  the  gel  of  iron  oxide  and  obtained  curves  similar  to  those  of 
silicic  acid.  The  water  content  decreases  continuously  with  the  decrease 
of  vapor  tension.  The  transition  point  is  less  pronounced  than  in  the 
case  of  silicic  acid  gel.  The  course  of  the  dehydration  and  the  read- 
sorption  is  shown  by  the  curves  in  Fig.  29. 


4 

A  Fresh  Gel— Dehydration 
Z  Fresh  Gel -Dehydration  and  Redehydratlon 

FIG.  29.    .Dehydration  of  Iron  Oxide  Gel  according  to  van  Bemmelen. 

The  curves  do  not  indicate  the  existence  of  a  hydrate  in  the  hydrogel. 
Colloidal  iron  oxide,  however,  goes  over  very  easily  into  crystalline 
hydrates,  %  for  example  Goethit  with  the  formula,  Fe203  •  H2O.  The 
water  content  remains  constant  even  up  to  79°  C.  It  differs  markedly 
from  the  gel  in  that  it  has  no  tendency  toward  adsorption.  This  is 
doubtless  due  to  the  finer  state  of  subdivision  and  to  the  reduced  surface. 

The  Adsorption  of  Arsenious  Acid.  —  Biltz  §  has  studied  the  ad- 
sorption combination  of  arseiiious  acid  with  colloidal  iron  oxide.  The 
complex  was  previously  given  a  definite  chemical  formula  and  was 
regarded  by  Bunsen,  who  discovered  that  the  hydrogel  of  iron  was  an 

*  C.  v.  Nageli  und  S.  Schwendener:  Das  Mikroskop.,  2.  Aufl.  Leipzig  (1877). 
C.  v.  Nageli:  Theorie  der  Garung,  121  ff.  Miinchen  (1879). 

t  van  Bemmelen:  Zeit.  f.  anorg.  Chemie,  20,  185-211  (1899). 

j  van  Bemmelen  und  E.  Klobbie:  Journ.  f.  prakt.  Chemie,  46,  497-529  (1892); 
Zeit.  f.  anorg.  Chemie,  20,  185-211  (1899).  H.  W.  Fischer:  Zeit.  f.  anorg.  Chemie, 
66,  37-52  (1910). 

§  W.  Biltz:  Ber.,  37,  3138-3150  (1904). 


170 


CHEMISTRY  OF  COLLOIDS 


antidote  for  arsenic  poisoning,  as  a  basic  ferriarsenite.  Biltz  has  shown, 
however,  that  the  curve  for  the  taking  up  of  arsenious  acid  by  colloidal 
iron  oxide  is  quite  similar  to  the  adsorption  curves  of  van  Bemmelen. 
We  have  to  do  here  with  an  adsorption  phenomenon  and  not  with  a 
chemical  compound,  Fig.  30.  The  arsenic  content  varies  with  the  con- 
centration of  the  arsenious  acid  in  solution,  as  may  be  seen  from  Fig.  30. 

E.   Colloidal  Aluminium  Oxide  and  Chromium  Oxide 

Aluminium  Oxide.  —  Compared  to  colloidal  iron  oxide  aluminium 
does  not  offer  anything  new.  Two  modifications  are  known,  one  of 
which  may  be  prepared  according  to  Crum  *  by  boiling  the  acetate. 
The  hydrosol  is  coagulated  by  acids  and  is  difficultly  soluble  in  an 
excess  of  the  reagent,  resembling  metairon  oxide  in  this  respect.  Small 
amounts  of  salts  are  necessary  for  precipitation,  and  it  is  not  reactive 


0.9 


0.5 


1.0 


2.0 
FIG.  30. 


3.0 


as  a  mordant  for  dyes.  Graham  has  called  this  the  meta  variety  of 
aluminium  oxide.  Graham's  f  hydrosol  is  made  by  dissolving  alumin- 
ium oxide  gel  in  a  solution  of  aluminium  chloride  and  dialyzing.  The 
product  is  very  sensitive  toward  electrolytes  and  reacts  as  a  mordant 
for  dyes  in  a  manner  similar  to  the  salts  of  aluminium.  It  is  also  sensi- 
tive to  concentrating  by  evaporation.  A  one-half  per  cent  solution  of 
the  hydrosol  may  be  boiled  without  coagulating,  but  on  evaporating  to 
one-half  the  volume  precipitation  takes  place  suddenly.  If  drops  of  the 
hydrosol  are  placed  on  red  litmus  paper  coagulation  sets  in  and  blue 
rings  are  formed  around  the  outside.  Well  dialyzed  hydrosols  may  be 
coagulated  by  most  electrolytes,  by  a  small  amount  of  spring  water, 
and  even  by  shaking  or  pouring  into  another  vessel.  Dilute  acids  coag- 
ulate them  but  the  precipitate  is  soluble  in  an  excess  in  contradistinc- 
tion to  the  meta  variety. 


*  W.  Crum:  Journ.  f.  prakt.  Chemie,  61,  390  (1854). 
t  Th.  Graham:  Liebigs  Annalen,  121,  41  (1862). 


COLLOIDAL  OXIDES  171 

A  reversible  colloidal  aluminium  oxide  has  been  made  by  A.  Miiller  * 
by  peptising  freshly  precipitated  aluminium  hydroxide  with  HC1.  A 
solution  of  aluminium  chloride  containing  1.124  g.  A1203  in  50  cc. 
was  treated  with  ammonia,  washed  as  quickly  as  possible  with  water, 
and  put  into  250  cc.  of  water  in  a  flask.  The  whole  was  brought  to  a 
boil,  and  20  cc.  of  N/20  HC1  added  drop  by  drop.  The  amount  of 
acid  added  is  only  about  7V  of  that  necessary  to  form  aluminium  chlo- 
ride. The  hydrosol  thus  prepared  is  quite  stable,  and  leaves  a  precipi- 
tate that  is  soluble  in  water.  If  the  amount  of  water  is  small  a  viscous 
liquid  is  formed  that  resembles  gum  arabic.  It  possesses  good  pro- 
tective power  and  has  a  gold  number  of  0.02  to  0.04.  The  stability  is 
probably  caused  by  the  large  amount  of  peptising  material  that  has 
not  been  removed  by  dialysis. 

Colloidal  Chromium  Oxide.  —  The  hydrosol  of  chromium  oxide  is 
made  similarly  to  that  of  colloidal  iron  oxide,f  by  the  peptisation  of  the 
hydroxide  by  means  of  chromium  chloride.  The  solution  is  deep  green 
and  is  somewhat  more  stable  than  that  of  aluminium  oxide.  Accord- 
ing to  H.  W.  Fischer  J  the  alkali  solution  of  the  hydrogel  is  also  a  colloid. 
Colloidal  chromium  oxide  should  be  more  closely  studied. 

The  older  belief  that  chromium  oxide  could  be  precipitated  by  the 
acetate  method  just  as  iron  oxide  is,  has  been  shown  by  Schiff  §  to  be 
only  partially  true.  Reinitzerlf  has  investigated  the  abnormal  be- 
havior of  chromium  in  this  regard,  and  the  results  are  of  importance  in 
analytical  chemistry.  He  found  that  chromic  salts  give  no  precipi- 
tate with  sodium  acetate,  but  on  the  contrary  the  presence  of  this 
element  could  in  some  degree  prevent  the  precipitation  of  iron  oxide  by 
the  acetate  method.  He  was  able  to  show  that  the  product  obtained 
by  boiling  pure  chromium  acetate  had  a  distinct  protective  effect  on 
the  precipitation  of  iron  oxide  by  alkalis  and  alkali  carbonate.  It  is 
not  a  stretch  of  the  imagination  to  assume  that  the  action  is  similar  to 
that  of  gelatin  on  colloidal  gold.  It  is  also  possible,  however,  to  ascribe 
the  phenomenon  to  the  formation  of  complexes  containing  iron,  chro- 
mium, and  acetate.  ||  The  question  has  not  yet  been  answered  experi- 
mentally. 

*  A.  Miiller:  Koll.-Zeit.,  2,  Suppl.  1,  VI-VIII  (1907);  Zeit.  f.  anorg.  Chemie, 
57,  312  (1908). 

t  Th.  Graham:  Liebigs  Annalen,  121,  52  (1862). 

J  H.  W.  Fischer  und  W.  Herz:  Zeit.  f.  anorg.  Chemie,  31,  352-358  (1902).  H.  W. 
Fischer:  Ibid.,  40,  39-53  (1904). 

§  H.  Schiff:  Liebigs  Annalen,  124,  168  ff.  (1862). 

1  B.  Reinitzer:  Sitzungber.  d.  Akad.  d.  Wiss.  Wien,  85,  11,  808-824  (1882). 

||  A.  Recoura:  Compt.  rend.,  129,  158-161,  208-211,  288-291  (1899);  Chem 
Centralbl.,  II,  416,  475,  523  (1899). 


172  CHEMISTRY  OF  COLLOIDS 

F.   Other  Colloidal  Oxides 

Colloidal  Tung stic  and  Colloidal  Molybdic  Acids 

Both  colloids  according  to  Graham  are  reversible  and  can  be  made 
by  dialyzing'the  acidified  solutions  of  the  corresponding  salts  of  sodium.* 
The  hydrosol  of  tungstic  acid  is  prepared  by  the  addition  of  a  slight 
excess  of  HC1  to  a  5  per  cent  solution  of  sodium  tungstate.  The  liquid 
is  put  in  the  dialyzer  and  HC1  added  every  two  days  until  all  the  alkali 
is  removed.  The  purified  acid  is  not  coagulated  by  acids,  salts,  nor  by 
alcohol.  The  residue  on  evaporation  is  in  the  form  of  glass-like  flakes 
that  stick  to  the  vessel  so  tenaciously  that  sometimes  it  is  impossible 
to  remove  them  without  breaking  the  surface  of  the  porcelain  dish. 
The  residue  may  be  heated  to  200  degrees  without  losing  its  solubility, 
but  in  the  neighborhood  of  red  heat  2  to  3  per  cent  of  water  comes  off  and 
the  mass  becomes  insoluble.  With  about  one-fourth  its  weight  of  water 
the  gel  forms  a  liquid  upon  which  glass  will  float.  With  sodium  car- 
bonate the  solution  effervesces.  The  taste  is  not  sour  but  bitter  and 
astringent.  It  prohibits  the  coagulation  of  liquid  silicic  acid  probably 
because  of  the  formation  of  silicon  tungstic  acid. 

It  is  difficult  to  prepare  the  colloid  free  from  alkali.  Sabanejeff  f 
was  inclined  to  believe  that  Graham's  preparation  was  a  tungstate,  but 
the  recipe  provides  for  the  removal  of  the  alkali.  L.  Wohler  J  dialyzed 
a  sample  made  by  Graham's  method  and  found  that  tungstic  acid 
diffuses  much  more  slowly  than  colloidal  molybdic  acid.  After  three 
weeks  dialysis  the  outer  liquid,  which  had  not  been  renewed,  contained 
about  one-third  as  much  tungstic  acid  as  the  inner  liquid.  When  left 
standing  for  13  months,  beautiful  double  refracting  crystals  of  tungstic 
acid  came  out.  Pappada  §  prepared  an  unstable  hydrosol  by  dialyzing 
a  solution  of  tungstic  acid  in  oxalic  acid.  This  sample  would  coagulate 
on  mere  warming.  Recently  Miiller  If  obtained  a  hydrosol  sensitive  to 
electrolytes  by  diluting  an  alcohol  ether  solution  of  WOCU,  with  water. 
Lottermoser  ||  has  made  a  sample  with  sodium  tungstate  supersatu- 
rated with  HC1.  According  to  the  concentration  hydrosols  are  obtained 
having  a  degree  of  dispersion  varying  from  suspensions  to  apparently 
homogeneous  solutions. 

*  Th.  Graham:  Poggendorffs  Annalen,  123,  539-540  (1864). 
t  A.  Sabanejeff:  Journ.  d.  russ.  phys.-chem.  Ges.,  27,  53  (1895);  29,  243  (1897). 
Ref.  Koll.-Zeit.,  3,  236  (1908). 

t  Lothar  Wohler:  Koll.  chem.  Beihefte,  1,  454-476  (1910). 
§  N.  Pappada:  Gazzetta  chimica  ital.,  32,  11,  22-28  (1902). 
If  A.  Miiller:  van  Bemmelen-Gedenkboek,  416-420  (1910). 
II  A.  Lottermoser:  Ibid.,  152-157. 


COLLOIDAL  OXIDES  173 

Colloidal  Molybdic  Acid.  —  This  colloid  may  be  made  according  to 
Graham  by  the  treatment  of  sodium  molybdate  with  HC1.  The  acid 
liquid  may  gelatinize  after  a  time  on  the  dialyzer,  but  becomes  liquid 
again  as  soon  as  the  salts  have  diffused  away.  After  repeated  addition 
of  HC1  and  diffusion  lasting  several  days  about  60  per  cent  of  the 
molybdic  acid  remain.  The  solution  is  yellow,  astringent,  acid  to 
litmus,  and  possesses  great  stability.  The  dried  residue  has  the  same 
appearance  as  that  of  tungstic  acid. 

All  investigators  have  not  been  successful  in  preparing  a  reversible 
molybdic  acid  hydrosol  according  to  the  method  of  Graham.  Bruni 
and  Pappada  *  were  unable  to  prepare  any  hydrosol  by  this  method 
because  the  molybdic  acid  diffused  through  the  membranes.  This  be- 
havior recalls  that  of  silicic  acid  at  times.  L.  Wohler  *  followed  the 
method  as  given  and  obtained  a  dilute  liquid  that  resembled  the  elec- 
trolytes in  that  it  diffused  through  parchment  paper,  although  much 
more  slowly.  Nevertheless  it  was  optically  inhomogeneous  and  had 
the  colloidal  property  of  being  precipitated  with  gelatin.  Graham's 
molybdic  acid  may  therefore  be  considered  a  semicolloid  and  represents 
that  class  of  substances  that  are  on  the  boundary  between  the  two 
great  divisions,  colloids  and  crystalloids. 

Both  tungstic  and  molybdic  acids  according  to  Graham  form  a 
number  of  crystallized  salts  with  alkalis.  These  can  be  coagulated  by 
heating  for  some  time  with  HC1.  According  to  Rosenheim  and  Ber- 
theim  *  the  dihydrate  dissolves  in  water  to  form  a  crystalloidal  solution 
capable  of  great  supersaturation.  The  hydrate  possesses  a  distinct 
solubility.  None  of  the  solutions  give  a  precipitate  of  molybdic  acid 
on  cooling. 

In  the  strongly  supersaturated  solution  the  same  two  authors  have 
made  molecular  weight  determinations  and  obtained  values  from  576 
to  610.  The  substances  in  solution  are  highly  dissociated  and  diffuse 
through  parchment  paper.  They  have  suggested  the  presence  of 
H2Mos025,  the  molecular  weight  of  which  would  be  1170.  The  smaller 
experimental  value  is  due  to  the  dissociation.  Rosenheim  and  David- 
sohn  *  found  that  on  concentrating  the  supersaturated  solution  at 
about  50  degrees  by  evaporation  a  well-crystallized  difficultly  soluble 
monohydrate  was  obtained.  If  the  concentration  took  place  under  20 
degrees  over  sulfuric  acid  a  glassy  residue  remained  that  contained  12.3 

*  G.  Bruni  e  N.  Pappada:  Atti  della  R.  Accad.  Lincei  Roma  (6),  354-358  (1900); 
Gazzetta  chimica  ital.,  31,  1,  244-252  (1901). 

t  Lothar  Wohler:  Koll.-chem.  Beihefte,  1,  454-476  (1910). 

t  A.  Rosenheim  und  A.  Bertheim:  Zeit.  f.  anorg.  Chemie,  34,  427-447  (1903). 

§  A.  Rosenheim  und  J.  Davidsohn:  Zeit.  f.  anorg.  Chemie,  37,  314-325 
(1903). 


174  CHEMISTRY  OF  COLLOIDS 

per  cent  of  water.  This  residue  was  soluble  in  water  and  formed  an 
opalescent  hydrosol,  from  which  a  precipitate  could  be  obtained  with 
alkalis  or  acids. 

Rosenheim  and  Davidsohn  conclude  from  their  experiments  that 
Graham's  acid  was  not  a  colloid  but  consisted  chiefly  of  the  dihydrate. 
The  molecular  weight  determinations  favor  this  point  of  view,  for 
Sabanejeff  *  obtained  620  while  Rosenheim  and  Bertheim,  as  already 
stated,  found  the  values  576  to  610.  Nevertheless  the  solutions  of 
these  investigators  and  those  of  Graham  do  not  appear  to  be  exactly 
alike  because  the  latter  obtained  his  product  on  the  dialyzer  while  the 
solutions  of  the  other  authors  mentioned  passed  freely  through  the 
membranes.  The  pronounced  propert}7  of  molybdic  acid  to  polymerize 
and  change  renders  the  existence  of  a  colloidal  modification,  having  the 
properties  of  Graham's  preparations,  not  improbable.  Too  great 
weight  .should  not  be  laid  on  the  molecular  weight  determinations  for 
similar  relations  exist  in  the  case  of  the  dyestuffs,  e.g.,  Congo  red.  The 
stability  in  the  presence  of  electrolytes  is  also  no  evidence  of  the  crys- 
talloidal  state,  because  there  are  a  great  many  reversible  colloids  that 
have  this  property  to  a  large  degree. 

Colloidal  Tungsten  and  Molybdenum  Blue.  —  Tungsten  and  mo- 
lybdenum blue  are  likewise  reversible  colloids.  They  were  employed 
by  Biltz  f  as  inorganic  dyes  in  an  investigation  that  proves  that  inor- 
ganic colloids  may  be  adsorbed  by  fibers  just  as  ordinary  dyestuffs. 
Silk  for  instance  is  dyed  directly.  Instructive  also  is  the  behavior  of 
molybdenum  blue  toward  animal  charcoal.  Although  the  former  is  a 
reversible  colloid  it  is  completely  adsorbed  by  the  charcoal  so  that  the 
filtrate  is  quite  clear.  Biltz  t  has  shown  that  both  these  two  colloids 
are  charged  negatively  and  therefore  precipitate  positive  colloids. 

Dumanski  §  has  determined  the  molecular  weight  of  the  crystalloidal 
blue  molybdenum  oxide  obtained  by  the  method  of  G.  Marchetti,1f  and 
has  found  the  value  440.  The  formula  is  supposed  to  be  Mo3O8  •  5  H20. 
When  this  solution  is  treated  with  salt  colloidal  molybdic  acid  is  formed. 

Preparation  of  Molybdenum  Blue.  —  Biltz  treated  the  acidified  solu- 
tion of  ammonium  molybdate  with  hydrogen  sulfide.  Dumanski  dis- 
solved 15  g.  of  ammonium  molybdate  in  400  cc.  water,  added  100  cc. 
of  3  to  4  nH2S04,  and  reduced  the  boiling  solution  with  hydrogen 

*  A.  Sabanejeff:  Ber.,  23,  R.,  87  (1890);  Journ.  d.  russ.  phys.-chem.  Ges.;  21, 
515-525  (1889). 

t  W.  Biltz:  Nachr.  d.  Kgl.  Ges.  d.  Wiss.  Gottingen,  Matb.-phys.  KL;  18-32  (1904), 
46-63  (1905). 

t  W.  Biltz:  Ber.,  37,  1095-1116  (1904). 

§  A.  Dumanski:  Koll.-Zeit.,  7,  20-21  (1910). 

H  G.  Marchetti:  Zeit.  f.  anorg.  Chemie,  19,  391-393  (1899). 


COLLOIDAL  OXIDES  175 

sulfide.  After  filtration  the  colloid  was  dialyzed  for  three  days.  Ac- 
cording to  Dumanski  crystallized  Mo03  can  be  obtained  by  boiling  a 
suspension  of  purified  Mo03  with  an  excess  of  metallic  molybdenum. 

Several  Other  Colloidal  Oxides 

Colloidal  vanadium  pentoxide  is  negatively  charged  and  according  to 
Biltz  *  can  be  made  by  treating  ammonium  vanadate  with  HC1.  The 
precipitate  is  washed  until  it  goes  into  solution  and  then  it  is  dialyzed. 
The  hydrosol  is  colored  yellow.  Concentrated  solutions  coagulate 
easily. 

Among  the  other  oxides  that  might  be  mentioned  are  manganese  f 
hydroxide  studied  by  van  Bemmelen,t  cobalt  oxide  prepared  by  Muller,§ 
bismuth,  lead,  ceri-copper  oxide,  etc.,  and  the  many  oxides  of  the 
platinum  group  studied  by  L.  Wohler  and  Ruff.lf 

Before  going  to  the  discussion  of  colloidal  sulfides  a  few  words  will 
be  devoted  to  a  group  that  plays  an  important  part  in  analytical  chem- 
istry. It  is  well  known  that  certain  organic  substances  containing  the 
hydroxyl  group  prevent  the  precipitation  of  iron  and  copper  hydroxides. 
This  property  is  possessed  by  sugar,  glycerin,  etc.  Graham  ||  showed 
by  dialysis  that  these  substances  formed  colloids,  and  that  the  oxide, 
or  saccharate  as  the  case  might  be,  remained  on  the  membrane,  while 
the  electrolytes  passed  through.  Such  colloids  have  been  made  by 
Grimaux**  by  means  of  manniterythrite,  glycerin,  tartaric  acid,  etc., 
in  a  sodium  hydroxide  solution. ft  Beside  the  iron  oxide  the  dialyzer 
contained  small  amounts  of  alkali  and  organic  substances,  that  doubt- 
less in  combination  with  the  oxide  formed  a  protective  colloid  and  pre- 
vented the  precipitation  of  the  remainder  of  the  iron.  Such  colloids  as 
these  cause  Lea's  colloidal  silver  to  be  soluble. 

*  W.  Biltz:  Nachr.  d.  Kgl.  Ges.  d.  Wiss.  Gottingen,  Math.-phys.  KL,  51  (1905). 

t  Journ.  Am.  Chem.  Soc.,  37,  1079. 

t  van  Bemmelen:  Journ.  f.  prakt.  Chemie  (2),  23,  324-349,  379-395  (1881). 

§  A.  Miiller:  Koll.-Zeit.,  2,  Suppl.  1,  VI-VIII  (1907);  Zeit.  f.  anorg.  Chemie, 
67,  315  (1908). 

H  Z.  B.  L.  Wohler  und  W.  Witzmann:  Zeit.  f.  anorg.  Chemie,  67,  323-352  (1908). 
O.  Ruff  und  F.  Bornemann:  Ibid.,  65,  429-456  (1910)  u.  a. 

I!  Th.  Graham:  Liebigs  Annalen,  121,  51  (1862). 

**  E.  Grimaux:  Compt.  rend.,  98,  1485-1488;  J.  B.  1884,  148. 

ft  H.  W.  Fischer:   Biochem.  Zeit.,  27,  311-325  (1910). 


CHAPTER  VIII 
COLLOIDAL   SULFIDES 

THE  tendency  of  the  sulfides  of  the  heavy  metals  to  go  through  the 
filter  while  washing  is  known  to  every  analytical  chemist.  In  this 
manner  larger  subdivisions  nearing  the  suspensions  are  obtained. 
Hydrosols  of  sulfides,  exactly  as  in  the  case  of  the  metals,  may  be  pre- 
pared in  all  possible  degrees  of  subdivision.  This  accounts  for  the 
want  of  uniformity  in  results  obtained  by  the  various  authors.  In 
fact  it  was  this  want  of  uniformity,  and  the  gradual  change  of  properties 
with  the  size  of  the  particles  that  led  to  the  discovery  of  the  discon- 
tinuity of  colloidal  solutions. 

The  straw-colored  liquid  that  is  obtained  by  conducting  H2S  into 
arsenious  acid  was  regarded  by  Berzelius  as  a  solution  of  arsenious 
sulfide.  Later  he  remarked  that  the  arsenious  sulfide  was  probably  in 
suspension  because  it  fell  out  on  standing.  Schulze,*  on  the  other 
hand,  obtained  solutions  which  were  so  stable  that  they  no  longer  gave 
a  precipitate  on  standing,  and  therefore  decided  that  he  had  hydrosols 
similar  to  those  described  by  Graham.  The  solutions  were  clear  in 
transmitted  light,  while  the  more  concentrated  showed  light  diffusion 
in  reflected  light.  Schulze  regarded  this  as  fluorescence. 

Colloidal  sulfides  have  been  the  subject  of  many  important  physico- 
chemical  investigations  especially  with  regard  to  coagulation.  Work- 
ing with  arsenious  sulfide  Picton  and  Linder  f  discovered,  and  Whitney 
and  Ober  t  elaborately  determined,  that  equivalent  quantities  of 
cathions  were  necessary  to  precipitate  the  colloid. 

Pure  colloidal  sulfides  are  as  a  rule  rather  unstable,  sometimes  even 
the  dilute  solutions  cannot  be  kept.  They  are  charged  negatively  and 
their  color  is  generally  that  of  the  precipitated  sulfide  itself.  Some  are 
yellow  while  others  are  dark  olive  green,  deep  brown  or  black. 

Colloidal  Arsenious  Sulfide 

The  hydrosol  of  arsenious  sulfide  was  known  to  Berzelius  but  was 
first  thoroughly  investigated  by  Schulze. f  The  latter  made  his  solu- 

*  H.  Schulze:  Journ.  f.  prakt.  Chemie  (2),  26,  431-452  (1882). 
t  H.  Schulze:  Journ.  f.  prakt.  Chemie  (2),  25,  431-452  (1882).     H.  Picton,  Ders. 
und  S.  E.  Linder:  Journ.  Chem.  Soc.,  61,  114-172  (1892);   67,  63-74  (1895). 
J  W.  R.  Whitney  und  J.  E.  Ober:  Zeit.  f.  phys.  Chemie,  39,  630-634  (1902). 

176 


COLLOIDAL  SULFIDES  177 

tions  by  conducting  H2S  into  a  solution  of  arsenious  acid.  He  dem- 
onstrated that  salts  of  univalent  cathions  had  the  least  pronounced 
precipitating  effect  while  the  trivalent  cathions  had  the  greatest.  The 
bivalent  occupied  an  intermediate  position.  According  to  Hardy  this 
law  has  an  application  in  the  case  of  the  precipitation  of  positively 
charged  colloids  by  anions.  As  already  stated  on  page  54  the  law  is 
not  without  exceptions,  especially  where  protective  effects  play  a 
part. 

Schulze  found  that  concentrated  solutions  gave  larger  particles  than 
the  more  dilute.  Picton  and  Linder  also  found  that  solutions  having 
different  degrees  of  dispersion  could  be  prepared.  They  obtained  four 
grades  which  they  called  a,  /3,  7,  5.  The  a  variety  contained  the  largest 
particles  while  6  was  the  finest  subdivision,  a  particles  could  be  de- 
tected with  an  ordinary  microscope  and  were  found  to  be  in  rapid 
motion,  the  Brownian  movement.  By  means  of  unglazed  porcelain 
filters  the  a  and  0  particles  could  be  easily  separated  from  the  liquid, 
while  d  particles  passed  through.  Solutions  of  the  latter  exhibited  the 
property  of  diffusion  and  osmotic  pressure.  In  this  series  of  experi- 
ments it  was  shown  for  the  first  time  that  the  properties  of  colloidal 
solutions  became  more  and  more  like  those  of  crystalloidal  solutions  as 
the  dispersion  became  greater.  However  even  the  finest  subdivisions 
of  Picton  and  Linder  revealed  themselves  to  be  inhomogeneous. 

Picton  and  Linder  also  demonstrated  that  during  the  precipitation  by 
electrolytes  a  portion  of  the  latter  was  carried  down  with  the  precipi- 
tate. In  other  words  negatively  charged  colloids  carried  down  cathions 
while  positively  charged  colloids  adsorbed  the  corresponding  amount  of 
the  anion  that  caused  the  coagulation.  In  the  case  of  arsenious  sulfide 
calcium  was  in  the  precipitate  when  calcium  chloride  was  employed  for 
the  coagulation,  and  a  corresponding  amount  of  hydrogen  ion  was 
found  in  the  liquid;  that  is  the  liquid  became  acid.  The  calcium  could 
not  be  washed  out  of  the  precipitate,  but  could  be  replaced  by  strontium, 
and  the  latter  in  turn  by  barium.  Van  Bemmelen  had  previously 
found  similar  relations  to  hold  for  soils. 

Whitney  and  Ober  showed  clearly  that  the  amount  of  the  various 
cathions  necessary  for  precipitation  was  exactly  equivalent.  This  re- 
sult is  of  theoretical  importance  because  from  the  amount  of  cathion 
held  in  the  precipitate  we  can  calculate  the  electric  charge  on  the 
particles  if  we  but  know  the  number  in  a  liter. 


178  CHEMISTRY  OF  COLLOIDS 

Other  Sulfide  Hydrosols 

A  large  number  of  colloidal  sulfides  of  the  heavy  metals  have  been 
prepared  by  Spring  *  and  Winssinger,  t  and  Schneider.  J  One  can  dis- 
tinguish three  different  methods  of  preparation. 

1.  Precipitation  of  the  sulfide  and  washing  with  water.     The  pepti- 
sation  takes  place  with  the  aid  of  H2S.     The  method  does  not  always 
lead  to  successful  results  for  the  sulfides  even  more  than  the  oxides 
have  the  tendency  to  form  precipitates,  and  in  this  state  they  cannot 
be  peptised.     In  this  regard  they  have  properties  midway  between  the 
oxides  on  the  one  hand  and  the  metals  on  the  other.    Their  reactions 
are  in  general  similar  to  those  of  the  metals. 

2.  A  second  method  is  given  by  Winssinger.     Very  dilute  solutions 
of  metal  salts  are  treated  with  H2S.     The  concentration  is  so  small  that 
the  amount  of  acid  formed  is  not  sufficient  to  cause  coagulation. 

3.  The  third  method  has  been  employed  by  Lottermoser  §  for  the 
formation  of  concentrated  solutions  of  colloidal  sulfides.     This  ingenious 
method  consists  in  the  employment  of  such  salts  that  the  products 
with  H2S  will  not  be  highly  dissociated.     For  instance,  concentrated 
solutions  of  colloidal  mercury  sulfide  were  made  by  the  use  of  mercury 
cyanide.     The  HCN  formed  is  of  course  very  weakly  dissociated  and 
therefore  cannot  have  a  great  precipitating  effect.     He  also  prepared 
colloidal  copper  sulfide  by  means  of  copper  glycocoll  and  H2S. 

The  treatment  of  oxides  by  H2S,  by  which  no  electrolyte  is  formed 
has  already  been  spoken  of  in  connection  with  the  preparation  of  arseni- 
ous  sulfide. 

Schulze  1f  prepared  antimonious  sulfide  by  treating  tartar  emetic  with 
H2S,  and  also  by  conducting  H2S  into  a  solution  of  antimonious  oxide 
in  tartaric  acid.  The  hydrosol  is  orange  red  and  can  be  obtained  in 
such  a  fine  state  of  subdivision  that  the  solution  is  clear  in  either  trans- 
mitted or  reflected  light.  Colloidal  cadmium  sulfide  was  made  by  Prost  || 
from  a  precipitate  of  cadmium  sulfide,  obtained  by  the  action  of  H2S 
on  an  ammoniacal  solution  of  cadmium  sulfate.  The  well- washed 
precipitate  was  suspended  in  water  and  then  treated  with  a  current  of 
H2S  whereby  the  cadmium  sulfide  went  into  colloidal  solution.  The 
color  was  golden  yellow  and  manifested  a  diffusion  of  light  rays  in  re- 
flected light. 

*  W.  Spring  et  G.  v.  Boeck:  Bulletin  de  la  Soc.  chim.  (2),  48,  165-170  (1887). 

f  C.  Winssinger:  Ibid.  (2),  49,  452-457  (1888). 

j  E.  A.  Schneider:  Ber.,  24,  2241-2247  (1891). 

§  A.  Lottermoser:  Journ.  f.  prakt.  Chemie  (2),  76,  293-306  (1907). 

If  H.  Schulze:  Journ.  f.  prakt.  Chemie  (2),  27,  320-332  (1883). 

||  E.  Prost:  Bull.  Acad.  Roy.  Belg.  (3),  14,  312-321  (1887). 


CHAPTER  IX 
COLLOIDAL  SALTS 

JUST  as  in  the  case  of  difficultly  soluble  metals,  sulfides,  and  oxides, 
almost  all  other  insoluble  bodies  may  be  obtained  in  the  colloidal 
form.  Of  importance  are  the  colloidal  salts,  which  may  be  prepared 
either  as  hydrosols  or  hydrogels.  Graham  *  obtained  colloidal  copper 
ferrocyanide  by  dissolving  the  brown  precipitate  in  ammonium  oxalate, 
and  subsequent  dialysis.  He  also  prepared  colloidal  Prussian  blue  by 
a  similar  method.  Schneider  |  obtained  a  hydrosol  of  ferric  phosphate, 
Lottermoser  and  E.  v.  Meyer  J  colloidal  silver  halides,  while  Lotter- 
moser  i  has  made  a  whole  series  of  hydrosols  of  difficultly  soluble  salts. 
Recently  the  number  of  these  hydrosols  has  been  greatly  increased  by 
the  work  of  von  Weimarn.§ 

Colloidal  Silver  Halides.  —  Hydrosols  of  silver  chloride,  bromide,  and 
iodide  were  prepared  by  Lottermoser  and  E.  v.  Meyer  If  by  treating  col- 
loidal silver  with  the  corresponding  halogen.  They  were  very  sensitive  to 
electrolytes.  Another  theoretically  interesting  method  has  also  been 
worked  out  by  Lottermoser.  1 1  It  is  based  on  the  effect  of  silver  nitrate  on 
the  silver  halides.  By  this  method  have  been  prepared  colloidal  solutions 
of  silver  chloride,  bromide,  iodide,  cyanide,  ferro  and  ferricyanides,  phos- 
phate and  arsenate.  The  reaction  is  particularly  interesting  because  it 
can  be  followed  by  measurement.  It  may  be  carried  out  in  two  ways. 
Either  a  measured  amount  of  silver  nitrate  is  treated  with  dilute  solu- 
tions of  the  alkali  halides,  or  silver  nitrate  from  a  buret  is  added  to  a 
known  amount  of  the  halide.  In  the  first  there  must  be  an  excess  of  the 
silver  ion  and  in  the  second  an  excess  of  the  halide  ion  in  order  to  obtain 
a  hydrosol.  If  too  much  of  the  lesser  constituent  is  added  precipitation 
results. 

1.  25  cc.  of  a  N/10  KBr  are  placed  in  a  small  flask  and  N/20  silver 
nitrate  solution  added  from  a  buret  with  violent  shaking.  A  hydrosol 

*  Th.  Graham:  Liebigs  Annalen,  121,  48  (1862). 
t  E.  A.  Schneider:  Zeit.  f.  anorg.  Chemie,  6,  84-87  (1894). 
j  A.  Lottermoser  und  E.  v.  Meyer:  Journ.  f.  prakt.  Chemie  (2),  66,  247  (1897). 
A.  Lottermoser:  Ibid.  (2),  57,  484-487  (1898);  68,  341  (1903)  u.  a. 
§  von  Weimarn:  Koll.-Zeit.,  2-5. 
If  I.e. 

||  A.  Lottermoser:  Journ.  f.  prakt.  Chemie  (2),  72,  39-56  (1905). 

179 


180  CHEMISTRY   OF   COLLOIDS 

of  silver  bromide  is  formed  having  opalescent  properties.  Shortly  be- 
fore the  bromide  ion  is  completely  converted  into  silver  bromide  the 
solution  is  turbid  and  not  very  stable.  When  about  50  cc.  of  the  silver 
nitrate  have  been  added  a  curdy  precipitate  comes  out. 

2.  The  second  method  is  the  opposite  of  the  first;  that  is  to  say, 
the  potassium  bromide  solution  is  in  the  buret  and  is  added  to  the 
solution  of  silver  nitrate.  The  phenomena  are  quite  analogous,  but 
the  hydrosols  differ  in  one  important  particular.  In  the  first  case  as 
long  as  the  halide  ion  is  in  excess  of  the  silver,  the  ultramicrons  are 
charged  negatively,  while  in  the  second  case  where  the  silver  ion  is  in 
excess  the  ultramicrons  are  charged  positively.  The  two  halide  hydro- 
sols mutually  precipitate  each  other,  as  is  to  be  expected  from  a  mixture 
of  oppositely  charged  colloids. 

By  the  use  of  formulas  such  as  we  have  employed  on  pages  75  to 
81  this  behavior  of  these  hydrosols  may  be  at  once  accounted  for.  In 
method  1  the  KBr  is  in  excess.  According  to  the  formula 
Ag+  +  Br~  — >  AgBr  insoluble  silver  bromide  is  formed  that  remains 
in  solution  as  ultramicrons.  The  negative  charge  is  the  result  of  the 
adsorption  of  bromide  ion  by  these  ultramicrons  in  a  greater  degree 
than  the  positive  ions,  such  as  the  potassium,  hydrogen  ion,  or  silver 
ion.  It  is  scarcely  to  be  doubted  that  the  bromide  is  responsible  for 
the  charge  because  coagulation  takes  place  at  once  as  soon  as  enough 
silver  has  been  added  to  unite  exactly  with  the  adsorbed  bromide  ion. 
From  this  it  does  not  follow  that  each  particle  of  silver  bromide  ad- 
sorbs one  bromide  ion  molecule.  In  fact  it  is  probable  that  the  ratio 
of  particles  to  adsorbed  ion  molecules  is  much  greater  at  the  beginning  of 
the  reaction  than  it  is  at  the  end.  In  other  words  a  silver  bromide  may 
adsorb  relatively  more  bromide  ion  when  the  concentration  of  the 
bromide  is  greatest.  Nevertheless  for  the  purposes  of  illustration  let 
us  assume  that  the  complex  is  represented  by  the  formula  [AgBr|  Br~. 
As  the  amount  of  silver  and  bromide  become  equivalent  at  the  end  of 
the  titration  the  ultramicrons  are  discharged  according  to  the  following 
equation : 


AgBr   -BrAg. 


This  presentation  is  by  no  means  a  complete  description  of  the  entire 
process,  for  in  point  of  fact  the  liquid  becomes  more  and  more  turbid, 
which  would  indicate  that  the  ultramicrons  are  uniting  among  them- 
selves to  form  larger  colloidal  complexes,  and  finally  a  flocculent  precipi- 
tate. It  is  natural  to  suppose  that  the  ultramicrons  would  grow  during 
the  titration  because  the  solution  is  constantly  being  supersaturated 
with  silver  bromide  formed  by  the  addition  of  silver  nitrate  to  the 
potassium  bromide.  The  ultramicrons  already  present  would  doubt- 


COLLOIDAL  SALTS  181 

less  act  as  nuclei  for  the  formation  of  fresh  silver  bromide.  Moreover 
where  the  silver  is  being  added  to  the  solution  of  potassium  bromide 
there  is  always  an  excess  of  silver  ion  that  would  neutralize  the  ultrami- 
crons  in  the  vicinity  and  thus  allow  them  to  unite  with  one  another. 
Further,  some  of  the  ultramicrons  will  become  positively  charged  be- 
cause of  the  excess  of  silver  ion,  and  therefore  the  oppositely  charged 
ultramicrons  would  neutralize  one  another  and  unite  to  form  larger 
particles  when  the  liquid  is  shaken.  A  microscopical  investigation 
would  give  additional  information  about  the  question. 

In  the  second  method  the  silver  ion  is  always  in  excess.  From  a  similar 
form  of  argument  we  have  the  ultramicrons  adsorbing  the  silver  ion 
and  therefore  charged  positively.  As  the  bromide  ion  is  added  the 
charge  is  neutralized  on  the  ultramicrons  and  coagulation  takes  place 
accompanied  by  processes  already  described  in  the  preceding  paragraph. 

Stability  of  the  Hydrosols  of  Silver  Halides.  —  The  hydrosols 
obtained  by  the  above  processes  are  not  very  stable;  they  coagulate 
on  standing,  sometimes  with  comparative  rapidity.  They  are  less  stable 
than  the  colloidal  metals.  Because  the  precipitate  has  no  appreciable 
electric  charge  the  neutralization  must  have  occurred  immediately  be- 
fore or  during  the  union  of  the  particles.  This  neutralization  must  be 
caused  by  the  giving  up  of  the  adsorbed  ion  molecules,  or  by  the  ad- 
sorption of  oppositely  charged  ion  molecules. 

Colloidal  Silver  Iodide  and  Other  Salts.  —  Lottermoser  prepared 
hydrosols  of  silver  iodide  by  the  same  method  used  for  silver  chloride 
and  bromide.  Silver  iodide  hydrosols  when  concentrated  are  very 
turbid  and  under  the  ultramicroscope  the  disperse  phase  appears  in 
the  form  of  rapidly  moving  bright  particles.  As  early  as  1902  the 
author  *  was  able  to  show  that  these  hydrosols  could  be  obtained  con- 
taining particles  that  are  amicroscopic.  The  solutions  are  made  by 
mixing  very  dilute  silver  nitrate  and  potassium  iodide.  These  hydro- 
sols are  particularly  suitable  for  observing  the  gradual  change  from 
amicroscopic  to  submicroscopic  particles. 

By  a  similar  method  Lottermoser  was  able  to  prepare  hydrosols  of 
silver  ferro  and  ferricyanide,  phosphate,  and  arsenate.  In  these  in- 
stances it  was  found  easier  to  prepare  hydrosols  by  allowing  silver 
nitrate  to  flow  into  the  solution  of  the  alkali  salt  in  question  than  it 
was  by  the  reverse  process.  These  solutions  differ  in  this  respect  from 
the  hydrosols  of  silver  halides.  Similar  conditions  hold  for  the  prepa- 
ration of  the  hydrosols  of  copper  or  iron  ferrocyanides. 

Photohaloids.  —  Silver  halides  possess  the  well-known  property  of 
turning  black,  when  exposed  for  a  sufficient  length  of  time  to  the  action 
*  R  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  149  (1905). 


182  CHEMISTRY  OF  COLLOIDS 

of  light.  Of  more  importance,  however,  is  the  property  of  accelerated 
reduction  by  suitable  reagents  on  short  exposure  to  the  light.  On  this 
principle  depends  the  art  of  photography.  The  white  plate  that  has 
been  exposed  contains  the  latent  image  and  the  substance  of  this  latent 
image  is  a  much  disputed  point  in  photography. 

That  bromine  is  liberated  from  silver  bromide  by  the  action  of  light 
is  an  undisputed  fact.  But  whether  the  reduction  forms  silver  or  a 
subbromide  is  a  matter  of  much  discussion.  By  treating  silver  chloride 
with  a  suitable  reducing  agent  Lea  *  obtained  colored  products  that  he 
named  photohaloids,  and  which  he  regarded  as  combinations  of  silver 
chloride  with  a  subchloride.  These  substances  corresponded  in  many 
ways  with  the  products  of  light  on  silver  chloride  and  silver  bromide, 
hence  these  were  included  under  the  name  photohaloids.  Baur  f 
demonstrated  that  photohaloids  could  be  made  by  treating  colloidal 
silver  with  the  halogens.  He  also  assumed  the  existence  of  subhalides. 
Abegg  J  has  shown,  however,  that  in  order  to  explain  the  properties  of 
light  exposed  silver  salts  it  is  merely  necessary  to  assume  that  a  portion 
of  the  salt  is  reduced  to  silver.  The  assumption  of  a  subhalide  is 
therefore  superfluous.  On  the  other  hand  several  authors,  notably 
Eder;§  have  defended  the  subhalide  theory.  This  theory  has  received 
some  support  from  the  work  of  Giintz  If  on  silver  subfluoride,  and  also 
from  some  electrochemical  investigations  of  Luther. || 

The  question  is  to  be  divided  into  two  others.  First,  do  subchloride 
bromides  and  iodides  of  silver  exist?  This  question  is  justified  because 
the  only  subhalide  of  which  we  are  sure  is  Ag2Fl,  made  by  Giintz,  and 
later  by  L.  Wohler  and  Rodewald.**  For  the  existence  of  other  sub- 
halides the  best  evidence  we  have  is  the  experiments  of  Luther,  which 
have  received  some  discredit  from  the  researches  of  Heyer,ft  and 
Weisz.Jt 

The  second  question  that  presents  itself  is,  what  is  the  necessity  of 
assuming  the  existence  of  a  subhalide  in  the  latent  image  or  in  the 
photohaloids?  This  question  must  be  answered  in  the  negative.  The 

*  M.  Carey  Lea:  Kolloides  Silber  und  die  Photohaloide.  German  ed.  by  Liippo 
Cramer.  Dresden  (1908). 

t  E.  Bauer:  Zeit.  f.  phys.  Chemie,  45,  613-626  (1903). 

t  R.  Abegg:  Wiedemanns  Annalen  d.  Physik,  N.  F.,  62,  428  (1897);  Archiv.  f. 
wiss.  Photographic,  1,  15  ff.  (1900). 

§  M.  Eder:  Photogr.  Korresp.,  276,  332,  463,  650  (1899);  Photogr.  Jahrbuch, 
80-87  (1900);  Sitzungsber.  d.  Akad.  d.  Wiss.  Wien,  114,  Ha,  1159-1193  (1905). 

If  Giintz:  Compt.  rend.,  110,  1337-1339  (1890). 

H  R.  Luther:  Zeit.  f.  phys.  Chemie,  30,  628-680  (1899). 
**  L.  Wohler  und  G.  Rodewald:  Zeit.  f.  anorg.  Chemie,  61,  54-90  (1909). 
ft  F.  Heyer:  Inaug.-Diss.     Leipzig  (1902). 
it  H.  Weisz:  Zeit.  f.  phys.  Chemie,  54,  305-352  (1906). 


COLLOIDAL  SALTS  183 

advocates  of  the  subhalide  theory  contend  that  if  the  photohaloids  were 
simply  mixtures  of  silver  and  the  silver  salt  the  silver  could  be  dis- 
solved out  by  nitric  acid.  On  the  basis  of  investigations  with  pyro- 
sols,  ruby  glass,  and  colloidal  solutions  R.  Lorenz  *  has  shown  that  the 
subhalide  hypothesis  is  unnecessary.  Luppo-Cramer  f  has  recently  in- 
vestigated the  objection  to  the  assumption  that  the  photohaloid  is  a 
mixture  of  silver  and  silver  halide,  and  has  come  to  the  conclusion  that 
the  objection  is  not  well  founded.  Luppo-Cramer  J  has  succeeded  in 
demonstrating  that  silver  bromide  may  protect  colloidal  silver  from 
the  attack  of  nitric  acid.  If  the  hydrosols  of  silver  and  silver  bromide 
are  mixed  and  concentrated  nitric  acid  immediately  added,  the  colloidal 
silver  will  be  dissolved.  If,  however,  the  mixture  is  first  coagulated  by 
sulfuric  acid,  a  colored  photohaloid  is  obtained  from  which  the  silver 
cannot  be  extracted  by  nitric  acid.  Likewise  the  same  author  has 
shown  that  colloidal  gold  cannot  be  dissolved  by  aqua  regia  if  the  gold 
is  first  precipitated  simultaneously  with  silver  bromide.  It  should  be 
further  remarked  with  regard  to  the  idea  of  removing  the  silver  from 
the  mixture  by  nitric  acid,  that  the  properties  of  colloidal  mixtures 
differ  widely  from  those  of  the  several  constituents  individually,  and 
often  resemble  chemical  combinations.  This  was  exemplified  very  well 
in  the  case  of  the  purple  of  Cassius.  Furthermore,  it  has  been  shown 
that  metastannic  acid,  copper  oxides,  and  many  others  cannot  be 
extracted  by  nitric  acid.  As  far  as  the  color  of  the  photohaloids  are 
concerned  it  should  be  remembered  that  it  corresponds  to  that  of  col- 
loidal silver,  even  when  mixed  with  the  silver  halides. 

One  could  form  a  rough  picture  of  the  protective  process  by  assum- 
ing that  the  particles  are  surrounded  by  the  silver  bromide  and  thus 
the  nitric  acid  could  not  reach  the  silver.  It  is  probable  that  the  process 
is  much  more  complicated  than  this,  and  that  its  true  nature  is  not  yet 
understood. 

It  is  interesting  that  gold  nuclei  can  cause  the  reduction  of  silver  in 
a  colloidal  silver  bromide  mixture,  just  as  they  can  in  a  suitable  homo- 
geneous solution  of  silver  salt  and  reducing  agent.  Such  observations 
have  been  made  by  Weisz§  and  also  in  the  author's  laboratory  by 
Thomse.  For  this  purpose  a  gold  nuclear  solution  is  employed  with  a 
hydrosol  of  silver  bromide  prepared  and  dialyzed  in  the  dark.  The 
hydrosol  is  put  into  two  small  vessels  to  one  of  which  is  added  the  gold 

*  R.  Lorenz:  Elektrolyse  geschmolzener  Salze,  2,  69  ff.     Halle  (1905). 

f  Luppo-Cramer:  Mitteilungen  in  der  Photogr.  Korrespondenz,  Koll.-Zeit.  und 
Monographien :  Photographische  Probleme,  117.  Halle  (1907);  Kolloidchemie  und 
Photographic,  70.  Dresden  (1908).  Das  latente  Bild,  Halle  (1911). 

t  Luppo-Cramer:  Photogr.  Korr.,  289  (1907);  397,  415,  520  (1909). 

§  Ibid.,  328. 


184  CHEMISTRY  OF  COLLOIDS 

nuclear  solution.  Both  solutions  are  now  precipitated  with  an  in- 
different electrolyte,  such  as  potassium  bromide.  If  now  a  suitable 
developer  is  added  to  both  vessels,  a  darkening  in  color  will  begin  al- 
most immediately  in  the  one  containing  the  nuclear  solution,  while  the 
other  will  remain  for  a  long  time  without  any  change.  Naturally  all 
these  operations  must  take  place  in  the  dark  room.  The  nuclear  solu- 
tion has  had  the  same  effect  as  exposure  to  light.  Nuclear  solutions  of 
silver  have  the  same  property.  From  this  it  follows  that  it  is  merely 
necessary  to  have  nuclei  present  in  order  to  account  for  the  develop- 
ment of  the  image  on  a  photographic  plate  by  the  developing  solution, 
and  the  photohaloids  can  therefore  be  regarded  as  mixtures  of  silver 
and  silver  halide.* 

To  account  for  all  the  properties  of  the  latent  image  much  more  com- 
plicated assumptions  are  necessary.  These  cannot  be  taken  up  here, 
but  it  is  quite  possible  that  a  colloidal  point  of  view  may  be  employed 
for  the  explanation  of  solarization  and  other  little  understood  phe- 
nomena. The  author  would  like  to  call  attention  to  a  not  widely 
known  experiment  of  Kogelmann,f  by  which  it  is  shown  that  with 
normally  exposed  plates  developed  in  acid  solution  containing  iron,  the 
substance  of  the  latent  image  lies  on  the  surface  of  the  granules,  while 
this  substance  is  in  the  granules  in  the  case  of  solarized  plates. 

Colloidal  Ferrocyanides.  —  The  most  simple  method  for  the  prepara- 
tion of  colloidal  ferrocyanides  is  to  pour  a  dilute  solution  of  the  metal 
in  question  into  a  solution  of  potassium  ferrocyanide.  Graham's 
method,}  which  is  to  dissolve  the  precipitated  cyanide  in  ammonium 
oxalate,  is  of  little  interest.  The  hydrosols  show  in  general  the  same 
color  as  the  hydrogels  obtained  from  them.  They  are  sometimes  quite 
homogeneous,  sometimes  filled  with  ultramicrons  that  are  in  rapid 
motion. 

Colloidal  copper  ferrocyanide  can  be  made  by  pouring  a  dilute  solu- 
tion of  copper  chloride  into  a  dilute  solution  of  potassium  ferrocyanide. 
The  hydrosol  thus  formed  is  clear  and  has  a  reddish  brown  color.  With 
increasing  concentration  of  copper  the  solution  becomes  more  and  more 
turbid  and  finally  such  concentration  relations  are  obtained  that  the 
hydrosol  coagulates.  This  relation  between  the  concentrations  is  not 
what  one  would  predict  from  a  knowledge  of  the  equivalents  of  the 
cupric  and  ferrocyanide  ions,  but  as  shown  by  Duclaux§  coagulation 

*  K.  Sichling:  Zeit.  f.  phys.  Chemie,  77,  1-57  (1911).  W.  Reinders:  Zeit.  f. 
phys.  Chemie,  77,  213-226  (1911). 

t  F.  Kogelmann:  Die  Isolierung  der  Substanz  des  photographischen  Bildes. 
Graz  (1894). 

t  Th.  Graham:  Liebigs  Annalen,  121,  48  (1862). 

§  J.  Duclaux:  Journ.  de  Chim.  Phys.,  6,  29-56  (1907). 


COLLOIDAL  SALTS  185 

takes  place  long  before  enough  copper  has  been  added  to  unite  exactly 
with  the  cyanide.  The  concentration  relations  just  before  coagulation 
occurs  have  been  called  by  Duclaux  the  "  point  critique."  A  small 
excess  of  copper  over  this  relation  causes  complete  precipitation,  while 
a  similar  excess  of  ferrocyanide  insures  the  stability  of  the  hydrosol. 

The  precipitate  contains  according  to  Duclaux  some  potassium  even 
in  the  presence  of  an  excess  of  copper,  and  the  formula  is 

(FeCy6)  CuTOKn 
where  m  +  —  =  2.     By  the  precipitation  with  an  excess  of  copper 

chloride  n  varies  between  1.3  and  0.2.  At  the  critical  point  where  the 
coagulation  first  begins  the  formula  for  the  precipitate  is  approximately 

(FeCy)  Cui.9Ko.2. 

Thus  it  is  seen  that  the  cupric  ferrocyanide  carries  down  a  part  of  the 
potassium  ferrocyanide,  and  that  the  latter  cannot  diffuse  into  the 
surrounding  liquid  nor  send  off  ion  molecules  because  of  dissociation. 
Duclaux  believes  this  potassium  ferrocyanide  is  chemically  combined. 
This  property  is  probably  connected  with  the  power  of  copper  ferro- 
cyanide to  form  semipermeable  membranes  for  crystalloids. 

When  the  potassium  ferrocyanide  is  in  excess  a  hydrosol  is  always 
formed  the  particles  of  which  are  charged  negatively.  From  this  we 
must  conclude  that  the  ferrocyanide  ion  is  adsorbed.  When  sufficient 
cupric  ion  has  been  added  to  unite  exactly  with  the  adsorbed  cyanide, 
coagulation  results.  The  reaction  may  be  expressed  by  the  following 
equation : 


|Cu 
fe 


Cu2FeCy6 
FeCy6 


FeCy6=  =  +  2  Cu- 


Cu2FeCy6 
K2FeCv6 


FeCy6Cu2 


For  the  cupric  ion  any  other  ion  in  equivalent  amount  may  be  substi- 
tuted to  cause  the  precipitation.  This  tendency  of  the  colloid  to  adsorb 
the  potassium  ferrocyanide  is  important  in  analytical  chemistry.  It  is 
evident  that  potassium  ferrocyanide  cannot  be  titrated  with  cupric 
chloride  because  the  end  point  will  be  reached  before  an  equivalent 
amount  of  cupric  ion  has  been  added.  A  great  many  other  precipi- 
tates behave  similarly  and  herein  lies  the  reason  that  zinc  salts  cannot 
be  titrated  accurately  with  potassium  ferrocyanide  or  sodium  sulfide. 

Other  Colloidal  Salts 

It  is  well  known  that  the  hydrosols  of  many  different  substances  may 
be  prepared  in  dilute  solution  by  means  of  chemical  reactions  in  a 
medium  in  which  the  colloid  is  insoluble.  This  is  the  method  followed 


186  CHEMISTRY  OF  COLLOIDS 

in  the  preparation  of  hydrosols  of  most  salts.  In  the  case  of  the 
more  soluble  salts  the  medium  must  be  chosen  so  that  the  solubility 
will  be  as  greatly  decreased  as  possible.  For  instance  the  solubility 
of  barium  sulfate  is  too  great  to  allow  the  preparation  of  the  hydrosol 
without  the  presence  of  a  protective  colloid.*  Neuberg  f  prepared  a 
hydrosol  of  barium  carbonate  by  conducting  a  current  of  C02  into  a 
solution  of  barium  oxide  in  methyl  alcohol.  The  gel  at  first  formed 
dissolves  spontaneously  pn  standing  in  methyl  alcohol.  In  a  similar 
manner  the  gels  of  barium  sulfate,  calcium,  and  magnesium  carbon- 
ate have  been  made.J  With  regard  to  the  preparation  of  organosols 
of  easily  soluble  salts  Paal  §  and  his  collaborators  have  given  many 
examples. 

Colloidal  Sodium  Chloride.  —  According  to  Michael  1f  chloracetic 
ester  and  sodium  malonic  ester  react  to  form  a  weakly  opalescent 
liquid.  He  considers  the  following  reaction  probable, 

COOC2H5 
/ 
C1CH2C02C2H5  +  CHNa         ->  CnH18O6ClNa 

COOC2H5 

whereby  an  addition  product  of  ethenyltricarbonic  ester  is  formed. 
Paal  has  shown  however  that  ethenyltricarbonic  ester  and  sodium 
chloride  are  formed  by  the  interaction  of  the  above  two  substances. 
The  clearness  of  the  solution  is  due  to  the  sodium  chloride  remaining 
in  solution  as  a  colloid.  The  substance  with  the  high  molecular  weight 
acts  as  a  protective  colloid  for  the  colloidal  sodium  chloride.  Petroleum 
ether  will  precipitate  the  colloid,  but  it  dissolves  again  with  unchanged 
properties  in  benzol.  In  a  similar  manner  Paal  and  Ktihn  have  prepared 
organosols  of  sodium  bromide  and  sodium  iodide.  The  employment  of 
protective  colloids  is  a  general  device  for  the  preparation  of  hydrosols. 
Lobry  de  Bruyn  ||  made  use  of  gelatin  in  the  preparation  of  not  only 
colloidal  metals,  but  also  a  series  of  salts  such  as  colloidal  silver  chloride, 
silver  chromate,  etc.  Paal  and  Voss  **  prepared  hydrosols  of  many  salts, 

*  R.  Zsigmondy:  Zur  Erkenntnis  der  Kolloide,  150  (1905). 

f  C.  Neuberg  und  E.  Neimann:  Biochem.  Zeit.,  1,  166-176  (1906).  Ders  und 
B.  Rewald:  Koll.-Zeit.  2,  321-324  (1908). 

J  C.  Neuberg:  Sitzungsber.  d.  Akad.  d.  Wiss.,  820-822.     Berlin  (1907). 

§  C.  Paal:  Ber.,  39,  1436-1441  (1906).  Ders.  und  G.  Kiihn:  Ber.,  39,  2859- 
2866  (1906);  41,  51-61  (1907).  Ders.  und  K.  Zahn:  Ber.,  42,  277-300  (1909). 

H  A.  Michael:  Ber.,  38,  3217-3234  (1905). 

||  C.  A.  Lobry  de  Bruyn:   Receuil  d.  travaux  chim.  des  Pays-Bas,  19,  236-249 
(1900). 
**  C.  Paal  und  F.  Voss:  Ber.,  37,  3862-3881  (1904). 


COLLOIDAL  SALTS  187 

among  others  silver  phosphate  and  silver  carbonate,  in  the  presence  of 
the  sodium  salt  of  prot-  or  lysalbinnic  acid.  Several  patents  have  been 
taken  out  for  the  preparation  of  such  colloids.  For  instance  the  chemi- 
cal works  of  Heyden  in  Radebeul  have  a  patent  for  the  preparation  of 
mercurous  halides  and  silver  chromate  soluble  in  water.  Other  patents 
are  held  by  Kalle  &  Co. 

Hydrogels  of  Difficultly  Soluble  Salts.  —  It  is  remarkable  that 
difficultly  soluble  salts  separate  out  in  the  form  of  a  gel  if  made  in  very 
concentrated  solution.  These  phenomena  have  been  noticed  by  Hart- 
ing,*  Buchner,f  Biedermann  {  and  Neuberg,§  and  especially  studied 
by  Weimarn.  If  As  an  example  of  this  may  be  cited  barium  sulfate 
jelly  that  may  be  obtained  by  mixing  concentrated  solutions  of  sul- 
focyanate  and  manganous  sulfate.  This  gel  gradually  turns  into  a 
crystalline  powder,  and  the  change  from  colloidal  particles  to  tiny 
crystals  has  been  followed  under  the  ultramicroscope  by  Weimarn.  If 
more  dilute  solutions  of  barium  sulfocyanate  and  manganous  sulfate 
are  mixed  a  granular  precipitate  is  formed  directly. 

*  P.  Halting:  Recherches  de  morphologic  synthetique  sur  la  production  arti- 
ficielle  de  quelques  formations  calcaires  organiques.  Amsterdam  (1872). 

t  G.  Buchner:   Chem.-Ztg.,  17,  878  (1893). 

J  W.  Biedermann:  Zeit.  f.  allg.  Physiol.,  1,  154-208  (1902). 

H.C. 

1f  P.  P.  v.  Weimarn:  Koll.-Zeit.  Bd.,  2-5,  in  Mitteilungen  "Zur  Lehre  von  den 
Zustanden  der  Materie." 


CHAPTER  X 

ORGANIC   COLLOIDS  * 

Colloidal  Organic  Salts 

COLLOIDAL  organic  salts,  important  both  in  technical  and  colloidal 
chemistry,  may  be  conveniently  divided  into  two  classes,  soaps  and 
dyestuffs.  The  two  classes  have  much  in  common,  nevertheless  they 
exhibit  great  differences. 

Soaps 

Under  soaps  we  will  understand  the  salts  of  the  higher  fatty  acids 
and  resins  with  acid  properties.  The  first  named  have  been  subjected 
to  elaborate  investigation  by  Krafft  f  and  his  students. 

Raising  of  the  Boiling  Point.  —  The  alkali  salts  of  fatty  acids  with 
low  molecular  weight  such  as  formic,  acetic,  and  propionic  acids  are 
dissociated  in  aqueous  solution  in  a  normal  manner,  and  are  of  no  great 
interest  for  the  present  work.  On  the  other  hand  the  salts  of  the 
higher  acids,  such  as  palmitic,  stearic,  lauric,  myristic,  and  oleic,  possess 
individual  properties  of  an  extraordinary  nature.  In  alcohol  they  be- 
have as  ordinary  crystalloids;  that  is  they  crystallize  out  on  evapo- 
ration, and  raise  the  boiling  point  corresponding  to  a  normal  molecular 
weight.  In  concentrated  aqueous  solution,  on  the  contrary,  they  are 
colloids;  that  is  they  do  not  raise  the  boiling  point,  become  gels  on 
cooling,  may  be  salted  out,  and  do  not  diffuse  through  membranes,  or 
at  least  very  slowly.  We  have  here  well-defined  chemical  substances 
that  are  either  crystalloids  or  colloids  according  to  the  solvent  employed. 
Salts  of  fatty  acids  lying  between  these  two  extremes  have  properties 
partly  colloidal  and  partly  crystalloidal.  This  can  be  seen  from  Table 
28. 

A  somewhat  greater  concentration  of  capronate  gives  no  raising 
of  the  boiling  point  and  the  solution  gels  on  cooling.  The  nonylate 
shows  an  appreciable  raising  of  the  boiling  point  that  becomes  relatively 
less  and  less  as  the  concentration  increases.  The  salts  of  the  higher 
fatty  acids  show  no  raising. 

*  Chapters  X,  XI,  XII. 

f  F.  Krafft  und  A.  Stern:  Ber.,  27,  1747-1761  (1894).  Ders.  und  H.  Wiglow: 
Ber.,  28,  2566-2582  (1895).  Ders.  und  A.  Strutz:  Ber.,  29,  1328-1334  (1896). 
Ders.:  Ber.  29,  1334-1344  (1896). 

188 


ORGANIC  COLLOIDS 


189 


TABLE  28 


Molecular  weight  calculated  from 

«             * 

Substance. 

Formula. 

100  gins. 

water. 

a 
Boiling 

Mol.  wt. 

a 

point. 

of  acid. 

b 

Sodium  acetate  

NaC2H3O2 

5      0.9 
1    25.2 

50.5) 
40.31 

82 

(    0.6 
1    0.5 

Sodium  propionate  

NaC3H6O2 

j      3.8 
1    19.8 

51.71 

46.2  j 

96 

j    0.6 
(    0.5 

f     3.5 

72.81 

{0.52 

Sodium  capronate  

NaC6Hn02 

J    20.6 
1    31.9 

77.9  1 
84.4  f 

138 

0.56 
0.61 

I   95.9 

98.  5J 

0.71 

Sodium  nonylate  

NaC9H17O2 

(      3.4 
\    20.4 

144.1  ) 
285.  5  f 

180 

j    0.8 
i    1.58 

(      1  A   A  \ 

ca.  1060 

~\            t 

ca.  4 

Sodium  palmitate  

NaCi6H3iO2 

)     10.4) 

i    25     \ 

Nearly 
infinite 

5    278    \ 

Nearly 
infinite 

f         1  £*             V 

ca.  1500 

)            ( 

ca.  5 

Sodium  stearate  

NaCi8H36O2 

(    16      ) 

i    27      \ 

Nearly 
infinite 

>    306    j 

Nearly 
infinite 

Sodium  oleate  

NaC18H3302 

26.  5  j 

Nearly 
infinite 

1  3°4  i 

Nearly 
infinite 

Kahlenberg  and  Schreiner  *  sought  to  explain  this  remarkable  fact 
by  the  assumption  that  the  solutions  of  soap  do  not  boil  normally. 
Careful  and  elaborate  experiments  of  Krafft  f  have  shown  that  the 
standpoint  of  Kahlenberg  and  Schreiner  is  not  well  taken,  and  that  solu- 
tions of  soap  do  boil  normally.  In  six  soap  solutions,  where  the  most 
concentrated  did  not  of  themselves  raise  the  boiling  point,  electrolytes 
such  as  sodium  chloride  gave  the  same  rise  as  they  do  in  pure  water. 
Krafft  also  showed  that  the  alcoholic  solution  of  sodium  oleate  gave  a 
normal  rise  as  seen  in  the  following  table. 

TABLE  29 


Gms.  alcohol. 

Gms.  sodium 
oleate. 

Rise  in  b.p. 

Mol.  wt.  (calcu- 
lated =  304.4) 

14.7 
14.7 
14  7 

0.5045 
1.2073 
1  9925 

0.131 
0.273 

301.3 
345.9 
Incompletely 

dissolved 

Moreover   Krafft  t   found   a   most  extraordinary  regularity  in  the 
gelatinization  of  soap  solutions,  the  temperature  being  almost  identical 

*  L.  Kahlenberg  und  O.  Schreiner:  Zeit.  f.  phys.  Chemie,  27,  552-566  (1898). 
t  F.  Krafft:  Ber.,  32,  1584-1596  (1899). 
$  Ibid.,  1596-1608  (1899). 


190 


CHEMISTRY  OF  COLLOIDS 


with  the  melting  point  of  the  pure  acids,  as  seen  in  Table  30.     From  this 
he  concludes  that  these  salts  must  be  partly  hydrolyzed.    This  is  sub- 

TABLE  30 


Sodium  stearate  

Ci8H35O2Na 

Melting  point  ca.  260° 

Stearic  acid  

OigHseOa 

«      «       69  4o 

Concentration  of  solution 

20%      15%      10%      1% 

Solidification  temperature 

69°          68°       68-67°    60° 

Sodium  palmitate 

Ci6H3iO2Na 

Melting  point  about  270° 

Palmitic  acid 

"           "          "        62° 

Concentration  of  solution  .  . 

20%                                1% 

Solidification  temperature  

62-61.8°                          45° 

Sodium  myristate  
Myristic  acid  

Ci4H27O2Na 

Melting  point  ca.  250° 
"   53.8° 

Concentration  of  solution  

20%                                1% 

Solidification  temperature  

53-52°                          31.5° 

Sodium  laurate 

Ci2H23O2Na 

Melting  point  ca  255-260° 

Laurie  acid 

"           "      "    43  6° 

Concentration  of  solution 

25%             20%            1% 

Solidification  temperature 

45-42°    ca.  36°        ca.  11° 

Sodium  oleate  

Melting  point  ca.  232-235° 

Oleic  acid  

Ci8H34O2 

«      '          «         «          Mo 

Concentration  of  solution  

25%                                1% 

Solidification  temperature  
Sodium  elaidate  

GuHi'iOiNa 

13-6°                                  0° 
Melting  point  225-227° 

Elaidic  acid 

"           "     45° 

Concentration  of  solution 

20%                                1% 

Solidification  temperature 

45.5-44.8°                       35° 

stantiated  by  the  fact  that  the  fatty  acid  may  be  extracted  by  certain 
solvents,  such  as  toluol,  from  concentrated  soap  solutions.  Further 
evidence  is  afforded  by  the  behavior  during  dialysis.  The  largest 
portion  of  that  diffusing  through  is  alkali  while  the  acid  remains  on 
the  membrane.  This  hypothesis  does  not  explain  the  want  of  any 
rise  in  the  boiling  point.  If  any  free  alkali  is  present  the  effect  on 
the  boiling  point  should  be  evident.  In  a  recent  and  thorough  investi- 
gation by  J.  W.  McBain  and  M.  Taylor  *  the  assumption  of  Krafft 
has  been  corroborated.  They  explain  the  absence  of  any  rise  in  the 
boiling  point  by  the  presence  of  air  in  the  soap  solution,  the  partial 
pressure  of  which  is  added  to  that  of  the  water  and  thus  causes  a  lower- 
ing in  the  boiling  temperature.  By  means  of  conductivity  measure- 
ments they  were  able  to  show  the  existence  of  an  equilibrium  between 
electrolyte,  colloid,  and  coagulum.  They  also  submit  evidence  which 
goes  to  show  that  the  equilibrium  may  be  expressed  by  the  equation: 
Soap  +  H20  +±  Acid  sodium  salt  +  NaOH. 

That  is  to  say  it  is  not  the  acid  that  is  set  free  by  hydrolysis  but  an 

acid  salt.     According  to  F.  G.  Donnan  and  A.  S.  White  no  well-defined 

chemical  compounds  are  formed  by  melting  a  mixture  of  sodium  palmi- 

*  J.  W.  McBain  and  M.  Taylor:   Zeit.  f.  phys.  Chemie,  76,  179-209  (1911). 


ORGANIC  COLLOIDS 


191 


tate  and  palmitic  acid.*  If  McBain's  assumption  with  regard  to  the 
air  does  not  hold  quantitatively  we  are  obliged  to  believe  that  the 
alkali  is  completely  adsorbed  by  the  colloidal  particles  of  the  acid  salt, 
free  acid  if  present,  and  by  the  unhydrolyzed  soap.  That  adsorption 
does  take  place,  and  that  sodium  hydroxide  and  its  chemical  combi- 
nations emulsify  the  higher  fatty  acids  has  been  shown  by  Donnan.  f 

Osmotic  pressure  measurements  give  just  as  abnormally  high  molec- 
ular weights  as  the  boiling  point.  For  sodium  oleate  Moore  and 
Parker  t  obtained  the  results  given  in  Table  31. 

TABLE  31 


Concentration, 
per  cent. 

T". 

Mol.  wt. 

0.5 
3.0 

55 
40 

7,100 
15,700 

Conductivity  measurements  by  Kahlenberg  and  Schreiner§  on 
dilute  soap  solutions  show  complete  hydrolysis  and  the  formation  of 
an  insoluble  acid  salt.  This  agrees  with  the  researches  of  McBain. 

The  colloidal  nature  of  soap  solutions  is  manifested  by  the  protective 
action  on  gold  solutions.  The  gold  number  of  sodium  oleate  lies  be- 
tween 0.5  and  2.  The  protective  effect  is  therefore  about  as  great  as 
that  of  gum  arabic,  and  much  smaller  than  that  of  gelatin.  The  pro- 
tective effect  of  sodium  stearate  increases  with  the  temperature. 

Detergent  Effect  of  Soap 

Regarding  the  detergent  effect  of  soaps  Spring  H"  has  carried  out  some 
interesting  experiments.  The  older  theories  of  Chevreul,||  Falck,** 
etc.,  attributed  the  detergent  effect  to  the  hydrolysis  of  the  soap,  or 
to  the  power  to  distend  the  fats,  and  also  to  the  emulsifying  effect. 
Spring  has  demonstrated  another  effect  of  the  soap.  There  are  a  great 
many  substances  free  from  fats  that  stick  fast  to  another  surface  also 
free  from  fats.  These  substances  cannot  be  washed  off  by  pure  water, 
but  if  soap  be  added  they  come  away  easily.  Examples  are  MnQ2, 

*  F.  G.  Donnan  und  A.  S.  White:  Journ.  Chem.  Soc.,  99,  1668-1679  (1911). 

t  F.  G.  Donnan:  Zeit.  f.  phys.  Chemie,  31,  42-49  (1899). 

j  B.  Moore  and  W.  H.  Parker:  Amer.  Journ.  of  Physiol.,  7,  261  (1902). 

§J.c. 

H  W.  Spring:  Koll.-Zeit.,  4,  161-168  (1909);  6,  11-17,  109-111,  164-168  (1910). 

||  M.  Chevreul:  Recherches  chimique  sur  les  corps  gras  d'origine  animale.  Paris 
(1910). 

**  R.  Flack:  Archiv.  f.  klin.  Chirurg,  73,  405-437  (1904).  Zeit.  f.  Elektrochemie, 
10,  834  (1904). 


192  CHEMISTRY  OF  COLLOIDS 

Fe2O3,  animal  charcoal,  etc.  This  property  on  the  part  of  the  soap  is 
closely  allied  to  the  tendency  to  lower  the  surface  tension.  The  soap 
piles  up  on  the  surface  of  the  particles  of  the  material  that  clings  and 
prevents  it  from  adhering  to  the  surface  in  question. 

Spring  washed  pine  soot  with  alcohol,  ether,  benzol,  and  finally  with 
benzol  vapors  in  order  to  free  the  soot  of  grease.  Soot  differs  in  its 
properties  after  this  treatment  in  that  it  will  form  a  stable  suspension 
in  water.  It  can  be  filtered  out  with  filter  paper  however.  If  the  soot 
is  mixed  with  a  1  per  cent  solution  of  soap  it  passes  through  the  filter 
paper,  not  even  so  much  as  staining  it.  The  soot  is  therefore  not 
strained  out  by  the  paper  but  is  adsorbed  on  the  surface.  If  the  filter 
containing  the  soot  is  turned  inside  out  and  water  passed  through,  the 
soot  remains  on  the  paper.  Soap  solution  will  remove  it  quite  easily. 
Spring  has  also  shown  that  the  soap  is  adsorbed  by  many  other  sub- 
stances beside  soot,  and  that  this  concentrating  of  the  soap  on  i;he 
surface  is  the  chief  factor  in  the  detergent  action.  It  seems  probable 
that  in  addition  to  the  adsorption  a  reaction  analogous  to  peptisation, 
that  is  a  charging  of  the  fibers  and  the  particles  of  soot  by  ion  adsorp- 
tion, takes  place. 

Interesting  also  is  the  corroboration  by  Spring  of  some  observations 
by  Donnan  and  Potts,*  that  the  stability  of  soot  suspensions  in  soap 
solutions  goes  through  a  maximum  as  the  concentration  of  the  soap  in- 
creases. This  maximum  is  situated  at  about  1  per  cent.  At  2  per  cent 
the  soot  sinks  to  the  bottom  almost  as  fast  as  in  pure  water. 

Emulsification  of  the  Fatty  Series 

Investigations  on  the  emulsifying  effect  of  the  sodium  salts  of  high- 
fatty  acids  on  the  members  of  the  fatty  series  have  been  carried  out  by 
Donnan. t  He  has  demonstrated  that  the  surface  tension  between 
water  and  fats  is  reduced  by  the  presence  of  soap.  This  reduction  is 
appreciable  from  the  sodium  salt  of  caprylic  acid  upwards  in  the  series. 
It  is  also  noteworthy  that  according  to  the  observations  of  Krafft,  and 
the  ultramicroscopic  investigations  of  Mayer,  Schaeffer,  and  Terroine,t 
the  colloidal  character  of  these  salts  begins  to  be  pronounced  with 
caprylic  or  lauric  acid.  Later  experiments  have  shown  that  the 
emulsifying  effect  also  begins  to  be  appreciable  with  laurates  and 
myristates.  This  effect  goes  through  a  maximum  in  the  case  of  laurates 
as  the  concentration  increases. 

*  F.  G.  Donnan  und  H.  E.  Potts:  Koll.-Zeit.,  7,  208-214  (1910). 

t  F.  G.  Donnan:  Zeit.  f.  phys.  Chemie,  31,  42-49  (1899). 

|  A.  Mayer,  G.  Schaeffer  et  E.  F.  Terroine:  Compt.  rend.,  146,  484-487  (1908). 


CHAPTER  XI 

DYESTUFFS 

SIMILAR  to  the  soaps  many  dyestuffs  are  salts  of  more  or  less  basic 
or  acid  properties,  and  possess  a  colloidal  nature.  They  are  often 
hydrolyzed  and  have  colloidal  tendencies  because  of  this.  How- 
ever in  cases  where  hydrolysis  plays  no  important  part  the  colloidal 
character  is  often  quite  pronounced,  for  instance  in  the  alkali  salts 
of  the  sulfo-acids.  In  general,  those  with  high  molecular  weights 
tend  to  go  into  the  colloidal  state.*  Many  of  them  do  not,  or  at 
least  very  slowly,  diffuse  through  membranes,  and  may  be  precipi- 
tated by  colloids  having  an  opposite  charge.  According  to  the  char- 
acter of  the  charge  they  migrate  in  a  potential  fall  either  to  the 
cathode  or  to  the  anode,  as  was  first  shown  by  the  researches  of 
Picton  and  Linder.f 

Perhaps  the  most  convincing  evidence  of  the  colloidal  nature  of  dye- 
stuffs  is  given  by  the  ultramicroscopic  observations  of  Raehlmann, 
Michaelis,  Hober,  and  others.  These  investigators  have  demonstrated 
that  solutions  of  dyestuffs  are  sometimes  filled  with  ultramicrons  even 
when  the  liquid  becomes  colorless  because  of  the  great  dilution.  In 
confirmation  of  this  view  are  the  experiments  of  Krafft  |  which  show 
that  many  of  these  salts  such  as  Methyl  Violet  raise  the  boiling  point 
of  water  correspondingly  less  than  that  of  alcohol. 

Most  of  the  work  done  on  dyestuffs  has  been  concerned  with 
practical  ^applications  in  the  textile  industry,  or  with  the  staining  of 
living  cells  in  microscopical  observations.  The  preparations  used  were 
those  obtainable  on  the  market  and  were  rarely  pure  chemical  sub- 
stances. This  disregard  for  strictly  scientific  principles  has  led  to  the 
accumulation  of  a  lot  of  contradictory  results.  On  the  other  hand  it 
should  be  considered  that  these  investigations  were  for  practical  pur- 
poses, and  it  is  precisely  with  these  impure  technical  preparations  that 
the  industries  have  to  do.  These  investigations  have  their  value  in 
technical  life  but  before  we  can  know  the  truth,  careful  work  must  be 
done  with  pure  substances.  A  good  start  in  this  direction  has  been 

*  W.  Biltz:  van  Bemmelen-Gedenkboek,  108-120  (1910). 
t  H.  Picton  and  S.  E.  Linder:  Journ.  Chem.  Soc.,  71,  568-573  (1897). 
%  F.  Krafft:  Ber.,  32,  1608-1622  (1899). 

193 


194  CHEMISTRY  OF  COLLOIDS 

made  by  Knecht,  Bayliss,  Biltz  and  v.  Vegesack.     Work  on  the  dye- 
stuffs  may  be  conveniently  divided  into  three  classes: 

1.  Those  which  have  to  do  with  the  state  of  the  dyestuff,  ultra- 
microscopy,  diffusion,  dialysis,  precipitation  with  electrolytes,  osmotic 
pressure,  and  with  the  conductivity. 

2.  Those  concerned  with  the  reactions  of  dyestuffs  toward  one  an- 
other, with  other  electrolytes,  and  with  positively  or  negatively  charged 
inorganic  colloids. 

3.  Those  which  have  to  do  with  technical  dyeing,  coloring  or  staining 
of  cells  or  bacteria. 

Ultramicroscopy  and  Dialysis  of  Dyestuffs.  —  After  Raehlmann  * 
had  shown  that  solutions  of  dyestuffs  were  often  filled  with  ultrami- 
croscopical  particles,  L.  Michaelis  f  undertook  an  elabroate  investigation 
and  endeavored  to  divide  the  solutions  according  to  the  appearance  in 
the  ultramicroscope.  His  classification  is  as  follows : 

1.  Those  completely  resolvable  under  the  ultramicroscope.     To  those 
belong  the  salts  of  sulfoacids  with  high  molecular  weights,  such  as 
Aniline  Blue,  water  soluble  Induline,  Bayrisch  Blue,  Violet  Black;  in 
addition,  certain  pseudo  solutions  such  as  Fuchsine  in  aniline  water, 
Fuchsine  in  sodium  chloride  (prepared  hot,  turbid  when  cold  turning 
violet  to  blue);  also  the  product  which  is  obtained  when  alcoholic  sol- 
utions of  Scharlack  are  poured  into  water. 

2.  The  partially  resolvable.    Those  containing  the  dissolved  substance 
in  two  phases;  the  optically  void,  and  those  in  the  form  of  ultramicrons. 
To  this  class  belong    concentrated  aqueous   solutions    of   Fuchsine, 
Methyl  Violet,  Neutral  Red,  Capri  Blue,  etc. 

3.  Those  not  resolvable,  as  Fluorescein,  Eosine,  Nile  Blue,  Methylene 
Blue,  Magdala  Red,  etc. 

Among  other  things  Michaelis  found  that  class  1  had  the  property  of 
dyeing  all  sorts  of  fibers,  while  the  third  class  stained  living  cells  par- 
ticularly well.  This  was  confirmed  by  Hober  and  F.  Kempner,  J  who 
were  engaged  in  the  adsorption  of  dyes  by  the  cells  in  the  kidneys  of 
frogs.  In  their  investigations  they  used  only  "lipoidunlosliche." 
They  found,  in  agreement  with  the  researches  of  Michaelis  and  Raehl- 
mann that  a  part  of  the  dissolved  substance  was  in  the  form  of  ultra- 
microns.  Indeed  solutions  of  dyestuffs  enumerated  in  class  1  were 
filled  with  ultramicroscopical  particles,  even  at  great  dilutions,  while 
other  dyestuffs  contained  scarcely  any. 

*  E.  Raehlmann:  Physikal.  Zeit.,  4,  884-890  (1903). 

t  L.  Michaelis:  Deutsche  med.  Wochenschr.,  1534  (1904);  Virchows  Archiv,  179, 
195-208  (1905). 

t  R.  Hober  und  F.  Kempner:  Biochem.  Zeit.,  11,  105-120  (1908). 


DYESTUFFS  195 

The  results  on  diffusibility  agree  with  those  obtained  by  the  ultra- 
microscope.  The  following  column  is  arranged  in  the  order  of  in- 
creasing diffusibility. 

Prussian  Blue,  Congo  Red,  Alkali  Blue,  3  B. 

Bavarian  Blue,  Aniline  Blue. 

Violet  Black. 

Nigrosine. 

Induline. 

Aniline  Orange,  Indigo  Carmine. 

Acid  Fuchsine. 

A  similar  series  was  obtained  from  the  results  of  precipitation  with 
CaCl2  and  NiCl2. 

Teague  and  Buxton  *  have  worked  with  the  diffusibility  through 
parchment  membranes.  They  distinguish  between  strongly  colloidal, 
medium,  and  weakly  colloidal  dyestuffs.  They  have  arranged  some 
basic  and  acid  dyes  in  the  following  ascending  order  of  diffusibility: 
Night  Blue,  Janus  Green,  Nile  Blue,  Neutral  Red,  Methylene  Blue. 

Acid  dyestuffs:  Congo  Red,  Trypan  Red,  Nigrosine,  Biebrich 
Scarlet,  Eosine,  Alizarine  Red. 

An  elaborate  investigation  by  Raehlmann  f  was  concerned  with  the 
ultramicroscopical  behavior  of  a  large  number  of  natural  and  artificial 
colored  substances,  and  some  of  their  reactions.  According  to  this 
work  Alkali  Blue,  Benzoblue  Black,  Congo  True  Blue,  water-soluble 
Chlorophyll,  and  colloidal  Indigo  belong  to  class  1.  As  in  the  case  of 
the  metals  the  color  of  the  ultramicrons  is  complementary  to  that  of  the 
solution  in  transmitted  light.  The  same  author  studied  the  reactions  of 
different  dyestuffs  on  one  another,  and  observed  in  many  cases  a  group- 
like  assembling  of  the  submicroscopical  particles.  He  further  observed 
that  on  being  mixed  with  solutions  of  egg  albumin  the  dyestuff  particles 
united  with  those  of  the  albumin  to  form  flocks.  It  is  therefore  pos- 
sible in  the  case  of  some  organic  colloids  to  follow  certain  reactions 
under  the  ultramicroscope  that  are  of  great  importance  in  colloidal 
chemistry.  It  is  also  worthy  of  note  that  Raehlmann  was  not  able, 
by  the  ordinary  microscope  and  transmitted  light,  to  detect  the  flocks 
formed,  even  when  the  size  of  the  flocks  attained  10  /z. 

Composition  and  Colloidal  Character  of  the  Dyestuffs.  —  If  one 
seeks  the  cause  of  the  dialysis  and  the  colloidal  character  of  the 
dyestuffs,  the  influence  of  the  size  of  the  molecule  is  at  once  appar- 

*  O.  Teague  und  B.  H.  Buxton:  Zeit.  f.  phys.  Chemie,  60,  469-488,  489-506 
(1907);  62,  287-307  (1908). 

t  E.  Raehlmann:  Archiv.  f.  d.  ges.  Physiol,  112,  128-171  (1906). 


196  CHEMISTRY  OF  COLLOIDS 

ent.  From  the  results  of  F.  Pfennigg,  Biltz  *  has  shown  with  about 
150  dyestuffs  that  the  number  of  atoms  rather  than  the  molecular 
weight  is  the  important  factor  in  the  diffusibility  through  membranes, 
He  defines  the  "  Molekiilgrosse "  (size  of  the  molecule)  according  to  the 
number  of  atoms,  and  finds  that  those  dyes  containing  up  to  45  atoms 
diffuse  quickly,  those  with  55  to  70  diffuse  slowly,  and  those  containing 
more  than  70  do  not  diffuse  at  all.  Certain  constitutional  influences 
override  the  effect  of  the  number  of  atoms.  Especially  is  this  true  of 
the  sulfo  group,  the  presence  of  which  increases  the  solubility  in  water 
and  the  diffusibility.  Dyestuffs  containing  'from  70  to  95  atoms 
ought  not  to  diffuse  through  parchment  paper,  but  will  quite  easily  if 
they  contain  a  sufficient  number  of  sulfo  groups.  For  instance  Alkali 
Blue  6  B  with  76  atoms  and  one  sulfo  group  does  not  diffuse,  while 
True  Acid  Violet  10  B  with  78  atoms  and  two  sulfo  groups  passes 
through  membranes  comparatively  easily.  The  Alizarine  group  on 
the  other  hand  retards  diffusion.  Some  alizarine  dyestuffs  with  only 
38  atoms  do  not  diffuse. 

Bredig  f  has  found  similar  laws  to  hold  for  the  rate  of  migration  of 
organic  electrolytes.  The  rate  falls  off  rapidly  with  increasing  number 
of  atoms,  then  more  slowly,  and  is  greatly  increased  by  the  presence  of 
the  sulfo  group. 

Osmotic  Pressure  and  Conductivity  of  Pure  Dyestuff  Solutions 

Dyestuffs  exist  that  have  no  power  to  diffuse  through  parchment 
membranes,  and  still  as  pure  substances  in  solution,  according  to  con- 
ductivity and  osmotic  pressure  measurements,  behave  as  electrolytes. 
To  these  belong  salts  of  type  of  Congo  Red,  Night  Blue.  Ultramicro- 
scopically  they  contain,  according  to  conditions  of  concentration  and 
presence  of  foreign  electrolytes,  amicrons  or  submicrons.  In  a  pure 
state  and  at  great  dilution  they  may  appear  optically  void.  Knecht  t 
first  drew  attention  to  the  fact  that  dyestuffs  in  a  high  state  of  purity, 
regarded  as  colloids  because  of  their  behavior  toward  parchment  mem- 
branes, show  a  normal  conductivity  and  raising  of  the  boiling  point. 
The  osmotic  pressure  of  a  pure  Congo  Red  solution  against  water,  ac- 
cording to  Bayliss,§  is  about  that  which  would  be  calculated  from  its 
molecular  weight.  By  the  boiling-point  method  Knecht  found  370  to 

*  W.  Biltz:  van  Bemmelen-Gedenkboek,  108-120  (1910). 

t  G.  Bredig:  Zeit.  f.  phys.  Chemie,  13,  191-288  (1894). 

J  E.  Knecht  and  J.  Batey:  Journ.  Soc.  of  Dyers  and  Colourists,  25,  No.  7  (July, 
1909). 

§  W.  M.  Bayliss:  Proc.  Roy.  Soc.,  91,  269-286  (1909);  Koll.-Zeit.,  6,  23-32 
(1910). 


DYESTUFFS  197 

412  for  the  molecular  weight  of  Benzopurple  instead  of  the  formula 
weight  756. 

In  order  to  free  Congo  Red  from  foreign  electrolytes  Bayliss  *  pre- 
cipitated it  with  HC1  and  dialyzed  in  a  parchment  osmometer  until 
free  from  electrolytes:  In  this  manner  he  obtained  a  blue  colloidal 
solution  of  the  free  acid  that  showed  a  very  small  rise  in  the  osmometer. 
To  get  the  sodium  salt  he  dialyzed  against  a  solution  of  sodium  hydroxide, 
which  caused  an  extraordinary  rise  in  the  osmometer.  The  excess  of 
alkali  was  removed  by  distilled  water.  After  a  week  of  dialysis  the 
column  retained  a  constant  height  of  50  mm.  When  the  ordinary  dis- 
tilled water  was  replaced  by  conductivity  water  free  from  C02  there 
was  a  further  rise  in  the  osmometer,  and  the  column  remained  at  97 
per  cent  of  what  it  should  from  calculation.  Only  a  faint  cloud  was 
to  be  seen  in  the  ultramicroscope.  The  extraordinary  sensitiveness  to 
carbonic  acid,  as  revealed  by  the  last  experiment,  has  also  been  observed 
by  Biltz  and  v.  Vegesack.f  In  these  experiments  membrane  hydroly- 
sis must  be  considered,  which,  according  to  Donnan,  plays  a  part  even 
in  the  case  of  strong  electrolytes. 

Bayliss  endeavored  to  determine  the  osmotic  pressure  of  a  series  of 
dyestuffs  that  diffuse  through  membranes  so  that  no  constant  pressure 
could  be  obtained  in  the  osmometer.  Aniline  Blue  with  a  molecular 
weight  of  734  does  not  diffuse  and  shows  only  one-half  the  theoretical 
pressure.  At  the  end  of  the  experiment  the  ultramicroscope  reveals  a 
large  number  of  bright  submicrons  in  the  diffused  cone  caused  by  the 
amicrons.  Bayliss  believes  the  solution  consists  of  single  molecules 
and  some  that  have  united  to  form  submicrons.  Endeavors  to  de- 
termine whether  or  not  the  osmotic  pressure  corresponded  to  the  num- 
ber of  particles  were  not  successful. 

An  elaborate  investigation  of  the  osmotic  pressure  of  Benzo  Purple, 
Night  Blue,  and  Congo  Red  was  undertaken  by  Biltz  and  v.  Vegesack. 
They  substantiated  the  results  of  Bayliss  that  the  osmotic  pressure  of 
pure  Congo  Red  against  water  is  smaller  than  that  calculated  from  the 
formula  weight.  They  also  showed  that  the  pressure  was  considerably 
less  when  dilute  solutions  of  electrolytes  were  employed  instead  of  pure 
water. 

Influence  of  Electrolytes.  —  Dyestuffs  of  the  Congo  Red  type  are 
extraordinarily  sensitive  toward  electrolytes.  As  shown  by  Bayliss 
mere  traces  of  electrolytes  are  sufficient  to  coagulate  the  particles  and 

*  W.  M.  Bayliss:  Proc.  Roy.  Soc.,  91,  269-286  (1909);  Koll.-Zeit.,  6,  23-32 
(1910). 

t  W.  Biltz  und  A.  v.  Vegesack:  Zeit.  f.  phys.  Chemie,  68,  357-382  (1909);  73, 
481-512  (1910). 


198  CHEMISTRY  OF  COLLOIDS 

materially  change  the  character  of  the  solution.     We  have  here  examples 
of  undoubted  electrolytes  that  are  just  as  sensitive  toward  salts  as  col- 
loidal solutions  of  the  metals. 
Just  as  in  the  case  of  carbonic  acid  neutral  salts  also  decrease  the 

N 
osmotic  pressure.    A  —  sodium  chloride  solution  caused  a  fall  of  207  to 

15  mm.,  and  when  the  sodium  chloride  was  removed  by  dialysis  the 
pressure  did  not  go  back  to  its  former  value.  Traces  of  the  alkali 
salts  are  sufficient  therefore  to  reduce  the  number  of  particles  to  a 
fraction  of  the  former  amount.* 

Biltz  and  v.  Vegesack  have  looked  into  this  matter  also  and  have 
shown  in  the  case  of  Benzopurple  that  the  raising  of  conductivity  of 
the  outer  water  from  566.1Q-6  to  786.10~6  by  the  addition  of  salt  was 
sufficient  to  reduce  the  osmotic  pressure  to  one-half  the  original  value, 
and  to  double  the  calculated  average  molecular  weight.  The  former 
solution  showed  only  a  few  particles  and  a  faint  light  cone,  while  after 
the  electrolyte  had. been  added  the  solution  manifested  a  pronounced 
light  cone  and  about  200  particles. 

Influence  of  Time.  —  As  has  been  already  stated  several  times 
hydrosols  change  their  properties  with  the  time.  Dyestuffs  are  par- 
ticularly suitable  for  demonstrating  this  phenomenon.  Biltz  and 
v.  Vegesack  made  solutions,  allowed  them  to  age  and  at  definite  in- 
tervals measured  the  conductivity  and  the  osmotic  pressure.  The 
latter  decreased  with  the  time,  and  the  ultramicroscopic  investigations 
showed  an  increase  in  the  optical  inhomogeneity.  Concentrated  solu- 
tions aged  much  more  quickly  than  the  more  dilute.  This  phenomenon 
has  been  observed  in  other  cases.  The  ageing  may  also  be  demon- 
strated by  measuring  the  inner  friction.  The  viscosity  increases  with 
the  time. 

Colloidal  Precipitation  of  Dyestuffs.  —  The  general  laws  governing 
the  mutual  precipitation  of  colloids  have  been  discussed  in  Chapter 
III.  Teague  and  Buxton  f  have  contributed  a  not  unimportant  addi- 
tion to  the  general  results  of  Biltz,  J  Bechhold,§  Neisser  and  Freide- 
mann.Tf  The  first  two  named  investigated  the  mutual  precipitation  of 
dyestuffs  and  also  the  effect  of  certain  organic  colloids  on  both  basic 
and  acid  dyestuffs.  In  general  the  law  of  precipitation  holds  here, 
that  oppositely  charged  colloids  precipitate  each  other  if  mixed  in  the 

*  J.  G.  Donnan  and  A.  B.  Harris:  Journ.  Chem.  Soc.,  99,  1554-1577  (1911). 
ti.c. 

%  W.  Biltz:  Ber.,  37,  1095-1116  (1904). 
§  H.  Bechhold:  Zeit.  f.  phys.  Chemie,  48,  385-423  (1904). 

\  M.  Neisser  und  U.  Friedemann:  Munch,  med.  Wochenschr.,  51,  465-469,  827- 
831  (1903-4). 


DYESTUFFS 


199 


proper  proportions.  Mixtures  outside  of  the  precipitating  zone  do  not 
cause  coagulation.  The  previous  generalizations  were  somewhat  en- 
larged by  the  discovery  that  the  precipitation  zones  became  larger  the 
less  pronounced  the  colloidal  character  of  the  dyestuff;  and  vice  versa, 
the  zones  became  smaller  the  slower  the  diffusion  of  the  dyestuff  in 
question.  Table  32  contains  a  number  of  examples. 


Janus  Green,  colloid,  basic, 


TABLE  32 

per  cent  is  precipitated  by  the  following  acid 
dyestuffs. 


Concentration 
in  per  cent. 


Dyestuff. 


Highly  colloidal. 


Congo  red. 


Nigrosine. 


Moderately  colloidal. 


Colloidal. 


Slightly. 


Sli 
col 


FJFTF 


+  represents  slight;  -\ — h  greater;  -\ — | — |-  pronounced  precipitation. 

The  numerous  tables  in  the  article  of  Teague  and  Buxton  *  go  to 
show  that  the  maximum  precipitation  occurs  when  the  two  dyes  are  in 
equivalent  amounts.  This  would  indicate  that  the  acid  and  basic 
dyes  in  the  solution  unite  to  form  a  salt  which  falls  out  because  of  its 
insolubility,  just  as  silver  iodide  is  precipitated  from  solutions  of  silver 
nitrate  and  potassium  iodide  mixed  together.  That  the  precipitation 
is  not  complete  when  one  or  other  of  the  substances  is  in  excess  has 
been  explained  in  the  discussion  on  sol  formation  of  silver  bromide, 
page  180.  When  the  cathion  and  anion  have  united  to  form  a  salt  the 
tendency  of  the  neutral  particles  is  to  join  together  in  complexes  which 
finally  precipitate  out.  If,  however,  one  or  other  of  the  dyes  is  present 
in  excess  the  salt  particles  adsorb  some  one  ion  more  than  another,  and 
thus  attain  a  positive  or  negative  charge.  The  charge  on  the  particles 
increases  the  stability  to  such  an  extent  that  little  or  no  precipitation 
may  result.  If  the  acid  dye  is  present  in  excess  the  particles  will 

*  l.c. 


200  CHEMISTRY  OF  COLLOIDS 

become  negatively  charged  because  of  the  adsorption  of  the  dyestuff 
anion,  and  vice  versa,  thus  forming  a  negative  or  a  positive  hydrosol 
respectively.  The  reactions  may  be  represented  by  the  following 
equation  where  DaNa  and  DcCl  represent  the  two  dyes. 

Dc+  +  Cl~  +  Da~  +  Na+  -»J,  DcDa  +  Na+  +  CL~. 

The  insoluble  combination  DcDa  would  first  form  ultramicrons  that 
would  grow  in  the  solution  until  a  flocculent  precipitate  would  be 
formed  unless  some  other  reaction  occurred  to  arrest  the  coagulation. 
If^  according  to  our  previous  scheme,  we  represent  the  ultramicrons  in 


the  solution  by   DC  •  Da  we  can  imagine  the  following  to  take  place : 


Adsorption  of  cathion  and  therefore  a  positive  hydrosol  is  formed 


DC  •  Da   +  Dc+?^  DC  •  Da  Dc+. 


Adsorption  of  the  anion  and  therefore  a  negative  hydrosol  is  formed 
I  DC- Dal  +  Da~<=*  I  DC -Da  I  Da~. 


The  colored  ion  of  highly  colloidal  dyestuffs,  as  is  well  known,  is 
adsorbed  by  very  many  substances  so  that  the  assumptions  underlying 
the  above  equations  are  not  at  all  strained.  The  wider  precipitation 
zones  of  dyestuffs  having  a  lesser  colloidal  character  may  be  explained 
on  the  basis  of  greater  solubility  of  the  dye  salt  formed,  and  also  on  the 
lessened  adsorption.  We  have  here  relations  similar  to  the  precipita- 
tion of  insoluble  inorganic  salts,  such  as  barium  sulfate,  magnesium 
ammonium  phosphate,  etc.,  where  sol  formation  is  difficult  because 
large  particles  form  so  easily. 

That  we  do  not  always  have  to  do  with  ionic  reactions,  where 
equivalent  quantities  unite  in  the  mutual  precipitation  of  dyestuffs,  is 
borne  out  by  the  fact  that  many  dyestuff  solutions  are  completely 
resolved  under  the  ultramicroscope.  Indeed  the  mutual  precipitation 
of  dyestuffs  is  analogous  to  that  of  oppositely  charged  colloids  and  to 
the  action  of  electrolytes  with  regard  to  precipitation.  The  author, 
some  years  ago,  observed  that  colloidal  gold  could  be  precipitated  by 
basic  dyestuffs.  Sometimes  where  the  concentration  relations  were 
suitable  the  precipitation  of  gold  and  Fuchsine  or  gold  and  Methyl 
Violet  left  a  perfectly  colorless  liquid  over  the  precipitate.  It  is  evi- 
dently not  a  case  here  of  chemical  action  between  Fuchsine  and  col- 
loidal gold.  It  was  also  discovered  that  the  precipitation  zone  was 
fairly  large,  and  that,  if  one  constituent  was  present  in  excess  of  the 
amounts  coming  in  the  zone,  a  protective  action  occurred.  See  page 
56.  The  Fuchsine  could  not  be  extracted  with  water,  but  it  dis- 
solved easily  in  alcohol.  The  insoluble  residue  if  pressed  with  an  agate 
pestle  had  a  metallic  luster,  and  therefore  consisted  of  gold. 


DYESTUFFS  201 

Protective  Effect  and  Dyestuff  Solutions 

Dyestuffs  as  Protective  Colloids.  —  Very  interesting  observations 
on  a  special  protective  effect  which  dyestuffs  have  on  solutions  of  col- 
loidal silver  bromide  have  been  made  by  Luppo-Cramer.*  Eder  f 
had  previously  demonstrated  that  the  adsorption  of  dyestuffs  by  the 
granules  of  silver  bromide  was  of  importance  in  the  sensitizing  of  silver 
bromide.  V.  Hubl  t  confirmed  and  extended  Eder's  discovery.  Luppo- 
Cramer  §  showed  that  Erythrosine  had  a  very  great  protective  effect 
on  silver  bromide  hydrosols.  Fifty  cubic  centimeters  of  the  hydrosol 
containing  about  0.2  per  cent  silver  bromide  were  completely  pro- 
tected by  1  cc.  of  Erythrosine  solution  (1  :  400)  from  the  precipitating 
influence  of  5  cc.  of  a  10  per  cent  solution  of  sodium  sulfate  or  potassium 
nitrate.  The  mixture  remained  unchanged  for  several  days,  while 
without  the  dye  precipitation  took  place  in  a  few  moments.  Similar 
protection  against  ammonia  has  also  been  observed.  The  spontaneous 
turbidity  of  silver  bromide  hydrosols  is  prevented  for  weeks  by  Erythro- 
sine, and  even  the  very  unstable  silver  chloride  hydrosols  may  be  kept 
for  a  long  time  if  they  are  but  colored  with  Erythrosine.  It  also  pro- 
tects the  hydrosols  against  coagulation  when  the  temperature  is  raised. 
The  pure  hydrosols  become  turbid  instantly  when  the  boiling  point  is 
reached,  but  if  they  are  colored  with  the  dye  they  may  be  boiled  with- 
out any  change  occurring.  Strangely  enough  these  crystalloidal 
Erythrosine  solutions  do  not  protect  gold  hydrosols,  according  to  ob- 
servations made  by  the  author.  It  is  therefore  clear  that  the  specially 
pronounced  protective  effect  of  Erythrosine  for  silver  bromide  must  be 
quite  different  in  character  from  the  protective  effect  of  certain  colloids 
such  as  gelatin  toward  gold  solutions.  The  protective  effect  on  silver 
bromide  is  probably  due  to  the  adsorption  of  the  anion  of  the  dye- 
stuff,  and  this  ion  prevents  somewhat  the  adsorption  of  cathion  neces- 
sary to  discharge  the  ultramicrons.  Beside  the  adsorption  of  the  dye- 
stuff  anion,  the  dyestuff  itself  is  doubtless  adsorbed.  ^ 

Dyestuffs  Protected  with  Colloids.  —  As  already  stated  on  page 
197  some  dyestuffs  are  very  sensitive  toward  electrolytes.  To  this 
group  of  dyes  belong  Congo  Red  and  Benzopurple,  which  may  be 
easily  coagulated  with  alkali  salts  even  when  the  disperse  phase  is  of 
molecular  dimensions.  Bayliss  ||  has  established  the  fact  that  such  dye- 

*  Luppo-Cramer:  Photographische  Probleme,  26-33.    Halle  (1907). 

t  M.  Eders  Handb.  d.  Photogr.,  3  (5  Aufl.),  152. 

J  A.  v.  Hubl:  Eders  Jahrbuch  fur,  189  (1894). 

§  Miss  Stevenson:  Koll.-Zeit.  (1912). 

1f  Zeit.  f.  Kolloidchemie,  10,  249  (1912). 

H  W.  M.  Bayliss:  Kail.  Zeit.,  6,  23-32  (1910). 


202  CHEMISTRY  OF  COLLOIDS 

stuffs  may  be  rendered  quite  as  stable  with  protective  colloids  as  gold 
solutions.  As  protective  colloid  for  Congo  Red  a  dialyzed  solution 
of  serum  albumin  was  employed.  The  dyestuff  alone  could  be  im- 
mediately precipitated  by  a  solution  of  N/100  calcium  sulfate,  but  in 
the  presence  of  the  protective  colloid  the  mixture  was  quite  stable. 
The  ultramicroscopic  investigation  revealed  the  fact  that  the  particles 
of  the  dyestuff  partially  coagulated  had  formed  a  complex  (probably 
due  to  adsorption)  and  were  thus  prevented  from  falling  out. 

Dyeing 

A  series  of  colloidal  chemical  investigations  is  concerned  with  the  be- 
havior of  dyestuff s  toward  fibers.  The  chief  point  at  issue  is  whether  the 
coloring  of  the  fibers  by  the  dye  is  a  chemical  process,  formation  of  a 
solid  solution,  or  purely  a  case  of  adsorption.  Of  a  large  number  of 
articles  only  a  few  will  be  mentioned,  namely  those  of  Georgievics,* 
Appleyard  and  Walker,  f  Biltz,  t  Freundlich  and  Losev.  §  In  these  it 
was  shown  that  the  adsorption  isotherms  are  just  as  applicable  to  the 
taking  up  of  dyes  by  fibers  as  by  charcoal.  In  other  words  the  process 
may  be  considered  as  one  of  adsorption,  upon  which,  in  specific  in- 
stances, solvent  and  chemical  action  are  superimposed. 1f 

V.  Georgievics  has  shown  that  the  taking  up  of  Benzidine  and  some 
other  dyestuffs  by  fibers  could  be  expressed  by  the  well-known  exponen- 
tial formula  || 

x 

—  =  «•  cn- 
m 

Biltz  found  similar  relations  to  hold  for  the  taking  up  of  inorganic 
colloids  such  as  molybdenum  blue,  'colloidal  iron  oxide,  vanadium 
pentoxide,  and  some  others,  by  silk  and  other  fibers.  The  adsorption 
curves  are  very  similar  to  those  obtained  by  dyeing  with  true  dyestuffs. 
He  found  further  that  the  fibers  might  be  replaced  by  aluminium 
hydroxide  without  causing  any  material  change  in  the  process,  but  that 
in  some  cases  the  dye  was  chemically  combined,  to  wit,  Alizarine  and 
colloidal  iron  oxide.  Appleyard  and  Walker  have  given  an  interesting 
example  of  the  differences  in  quantitative  relations  between  the  ad- 
sorption of  dyes  by  fibers  on  the  one  hand  and  a  chemical  union  of  the 

*  G.  v.  Georgievics:  Monatshefte  f.  Chemie,  16,  705-717  (1894).  Ders.  und 
E.  Lowy:  IUd.,  16,  345-350  (1895);  Sitzungsber.  d.  Akad.  d.  Wiss.  Wien,  104,  lib, 
309-314  (1895). 

t  J.  Appleyard  and  J.  Walker:  Journ.  Chem.  Soc.,  69,  1334-1349  (1896). 

%  W.  Biltz:  Her.,  37,  1766-1775  (1904);  38,  2963-2977  (1905). 

§  H.  Freundlich  und  G.  Losev:  Zeit.  f.  phys.  Chemie,  69,  284-312  (1907). 

fl  W.  Borsche:  Med.  Naturwiss.  Archiv,  2,  411-422  (1910). 

||  H.  Freundlich:  Zeit.  f.  phys.  Chemie,  67,  392  (1907). 


DYESTUFFS 


203 


dye  with  a  crystalloid  on  the  other.     If  silk  is  colored  with  a  solution 

of  picric  acid  in  water  the  equilibrium  may  be  expressed  by  the  ex- 
ponential  function   given   above. 

The  curves  are  given  in  Fig.  31. 

If  the  silk  is  replaced  by  diphenyl- 

amine,   which   forms   a    chemical 

compound   with   the   picric   acid, 

none   of   the   latter   is   taken   up 

until  the  concentration  has  reached 

13.8  g.  of  the  acid  to  one  gram 

water.    If  more  acid  is  added  the 

diphenylamine  takes  it  up  until  all 

the  latter  is  used  up,  while  the  con- 

centration  of  the  solution  remains 

constant   as   long  as  there  exists 

any  diphenylamine.   From  now  on 

the  concentration  of  the  solution 

increases  until  the  saturation  point 

is  reached.    Fig.  32  represents  the  *% 

course  of  the  reaction. 

In  order  to  explain  the  curve  by 

the  assumption  of  a  chemical  com- 
bination, one  must  regard  the  solid  phase  as  variable.     One  may  also 

assume  that  the  solid  is  a  solid  solution.  This  is  somewhat  of  a  dis- 
tortion as  shown  by  Freundlich; 
for,  first  of  all  the  adsorption 
equilibrium  is  reached;  and  sec- 
ondly, the  adsorption  of  all  sorts 

•— ~  of  substances  follows  the  same 

curves  where  chemical  combina- 
tions are  quite  impossible. 

Freundlich  and  Losev  have 
thoroughly  studied  the  taking 
up  of  both  basic  and  acid  dye- 


FIG.  31. 


FIG.  32. 


stuffs  (Crystal  Violet,  New 
Fuchsine,  etc.)  and  found  that 
the  adsorption  of  basic  dyestuffs  differs  from  that  of  the  acid.  In 
the  case  of  the  basic  dyestuffs  chemical  action  as  well  as  adsorp- 
tion takes  place.  The  chemical  action  occurs  on  the  surface  of  the 
charcoal  without  the  latter  taking  any  part.  The  dyestuff  salt 
adsorbs  only  the  basic  dye  while  the  acid  remains  quantitatively  in 
solution.  The  base  suffers  a  change,  probably  into  an  isomeric  modi- 


204 


CHEMISTRY  OF  COLLOIDS 


fication,  becomes  insoluble  in  water,  but  can  be  dissolved  in  benzol  or 
alcohol.  According  to  Hantzsch  *  such  transitions  also  occur  when 
solutions  of  triphenylmethane  dyestuffs  are  treated  with  alkalis.  An 
especially  good  case  is  that  of  Crystal  Violet  (hexamethyltriamido- 
triphenylmethanechlorhydrate) . 


CH3N- 


-  N(CH3)2 
/y  (CH3)2 

=  MXCI 


When  to  one  mol.  of  the  dyestuff  one  mol.  of  potassium  hydroxide  is 
added  the  Cl  is  replaced  by  OH  and  a  true  soluble  base  is  obtained. 


(CH3)2N 


-  N(CH3)2 
^  (CH3)2 

NXOH 


This  base  like  the  dyestuff  is  colored,  shows  a  basic  reaction,  and  is 
therefore  dissociated  in  solution.  Gradually  the  solution  becomes 
colorless,  does  not  react  basic,  and  contains  the  ordinary  dyestuff  base 
(Pseudo  base)  or  carbinol. 


(CH3)2N  - 


-  N(CH3)2 

-  N(CH3)2 


During  adsorption  something  else  occurs.  The  adsorbed  dyestuff  has 
the  same  color  as  that  of  the  solution,  but  the  surface  of  the  charcoal 
has  a  brownish  luster.  The  adsorbed  material  gives  a  brown  solution 
in  benzol,  while  the  solution  colors  paper  or  alcohol  violet.  A  further 
or  different  change  must  have  taken  place. 

It  is  possible  that  a  similar  colored  base  is  formed  when  wool  is  dyed 
by  Fuchsine.  The  well-known  experiment  where  the  colorless  Fuch- 
sine  base  (pseudo-base)  colors  wool  deep  red  may  be  explained  by 
some  such  change.  Usually  the  red  color  is  attributed  to  the  forma- 
tion of  a  salt  by  the  union  of  the  base  with  the  wool.  Such  a  salt 
formation  is  not  improbable  because  animal  fibers  contain  amido  and 
carboxyl  groups  that  might  unite  with  the  dyestuff  base.  Suida  and 
Gelmo  f  favor  this  view,  according  to  which  the  reaction  is  an  esteri- 

*  A.  Hantzsch  und  G.  Osswald:  Ber.,  33,  278-317,  752-760  (1900). 
t  W.  Suida  und  P.  Gelmo:    Monatshefte  f.  Chemie,  26,  855-878  (1905);    27, 
225-235,  1193-1198  (1906). 


DYESTUFFS  205 

fication.  In  the  light  of  the  existence  of  the  adsorption  isotherms  it  is 
still  questionable  whether  the  fast  dyeing  of  wool  and  silk  is  due  to 
chemical  reactions  or  to  mere  adsorption.  In  favor  of  the  latter  view 
is  the  fact  that  vegetable  fibers  take  up  dyestuff  salts,  such  as  Benzidine, 
and  others.  There  seems  to  be  less  likelihood  of  chemical  reactions 
taking  place  here.  The  researches  of  Weber  *  point  toward  adsorption. 
According  to  these  investigations  the  dyeing  of  vegetable  fibers  de- 
pends upon  the  structure  of  the  latter.  Cotton  fibers  are  flattened 
tubes  with  thick  walls.  These  walls  are  not  colored  by  the  dye,  but 
the  latter  penetrates  them  and  is  precipitated  in  the  tiny  tubes.  Fibers 
that  have  no  central  canal  (dead  fibers)  are  not  colored  by  the  dye. 
Weber  has  given  further  evidence  of  this  peculiar  behavior  on  the 
part  of  fibers.  Dinitrocellulose  can  be  colored  just  as  the  ordinary 
cellulose.  If  the  nitrocellulose  is  dissolved  and  the  solvent  evaporated 
away  the  residue  is  not  colored  by  the  dye. 
Georgievics  f  has  recently  stated  that  for  the  taking  up  of  acid  and 


basic  dyestuffs  in  very  dilute  solution  ^^  is  a  constant.     That  is  to  say 

(-/acid 

Henry's  law  obtains  in  very  dilute  solution.  That  the  same  law  holds 
for  the  condensation  of  gas  on  the  surface  of  charcoal  at  low  pressure 
has  long  been  known.  |  It  is  probable  that  we  have  to  do  here  with  a 
general  law. 

Capillary  Analysis 

As  shown  by  Schoenbein,§  dissolved  substances  rise  to  different 
heights  in  strips  of  unglazed  paper.  Usually  the  water  precedes  the 
dissolved  substance  but  the  height  of  the  latter  is  not  always  the  same. 
Strips  of  filter  paper  were  hung  in  three  solutions  containing  turmeric 
dye.  The  first  solution  contained  sodium  hydroxide,  the  second  barium 
hydroxide,  and  the  third  calcium  hydroxide.  The  brown  color  rose 
seven-tenths  of  the  height  of  the  water  in  the  first  vessel,  three-tenths 
in  the  second,  and  one-tenth  in  the  third.  Similar  results  were  obtained 
with  solutions  of  dyestuffs;  the  dye  followed  the  water  up  the  paper 
but  to  different  heights. 

As  pointed  out  by  W.  Ostwald^  the  cause  is  to  be  sought  in  the 

*  C.  O.  Weber:  Lehnes  Farberzeitung,  6,  161,  184,  201,  214  (1893-4). 

t  G.  v.  Georgievics:    Sitzungsber.  d.  Akad.  d.  Wiss.  Wien,  120,  lib,   857-869 


t  M.  W.  Travers:  Zeit.  f.  phys.  Chemie,  61,  241-248  (1908).  W.  McBain: 
Ibid.,  68,  471-497  (1910).  A.  Titoff:  Ibid.,  74,  641-678  (1910).  Ida  Fr.  Homfray: 
Ibid.t  74,  129-201  (1910). 

§  C.  F.  Schoenbein:  Verb.  d.  Naturf.-Ges.  Basel,  III  Teil,  249-255  (1863). 

H  W.  Ostwald:  Lehr.  d.  allg.  Chemie  (2  Aufl.),  1,  1095.    Leipzig  (1905). 


206 


CHEMISTRY  OF  COLLOIDS 


adsorption  of  the  various  materials.  Goppelsroeder  *  has  carefully 
studied  these  relations  and  has  devised  an  analytical  separation  that 
has  a  wide  application  in  work  on  dyestuffs,  alkaloids,  and  oils.  An 
investigation  by  Fichter  and  Sahlbom  f  has  shown  that  negatively 
charged  colloids  rise  in  strips  of  paper  and  the  positively  charged  are 
precipitated  at  the  dipping  surface.  The  authors  attribute  the  precipi- 
tation to  the  presence  of  currents  in  the  capillaries  of  the  paper.  In 
glass  capillaries  also  the  water  rises  higher  than  basic  dyestuffs  and 
positive  colloids. 

TABLE  33 
From  Pelet-Jolivet 


Effect  of 

Electrical  charge  by 
contact.     (Perrin.) 

Coagulation. 

Coloring  of  fibers. 

Rise  in  capillaries. 

Acids  
Bases  

Salts  . 

Increase  charge 
on  positive 
surfaces 

Increase  sta- 
bility of  pos- 
itive colloids 

Decrease  col- 
oring by 
basic  dyes 

Increase  rise 
of  basic  dyes 

Decrease  charge 
on  negative 
surfaces 

Coagulate  neg- 
ative colloids 

Increase    col- 
oring by 
acid  dyes 

Decrease  rise 
of  acid  dyes 

All  acids  having  the  same  H  ion  concentration  work  alike 

Bases  have  the  reverse  effect  (exception  in  case  of   acid  dyes) 

Ions  with  the  op- 
posite sign  re- 
duce charge  on 
a  surface 

Ions  of  opposite 
charge  cause 
coagulation 

Ions  with  op- 
posite 
charge  in- 
crease col- 
oring 

Ions  with  op- 
site  charge 
decrease 
rise 

May  reverse 
charge 

Colloid  may 
become  oppo- 
sitely 
charged 

Higher  the  val- 
ence the 
greater  the 
effect 

The  ions  with  higher  valence  have  the  greatest 
effect 

In  general  ions 
with  higher  val- 
ence do  not  in- 
crease the 
charge  on  a 
surface  having 
the  same  sign 

Ions  of  same 
sign  increase 
stability 

Ions  with 
same  sign 
decrease 
coloring 

Ions  with 
same  sign 
increase  rise 
of  basic 
dyes  but  not 
of  acid  dyes 

*  F.  Goppelsroeder:  Kapillaranalyse.     Basel  (1906). 
t  F.  Fichter  und  N.  Sahlbom:  Koll.-Zeit.,  8,  1-2  (1911). 


DYESTUFFS  207 

Elaborate  investigations  by  Pelet-Jolivet  *  and  his  collaborators  re- 
sulted in  the  discovery  of  interesting  relations  between  the  rise  in 
capillaries,  on  the  one  hand,  and  dyeing,  coagulation,  and  electrical 
charge  by  contact,  on  the  other.  Some  of  these  relations  are  presented 
in  Table  33. 

The  results  in  the  table  may  be  summarized  as  follows:  All  the  in- 
fluences that  tend  to  discharge  the  particles  increase  the  power  of  the 
dye  to  color  the  fiber  and  decrease  the  rise  in  capillaries.  On  the  con- 
trary, influences  tending  to  increase  the  stability  of  the  colloidal  dye- 
stuff,  increase  of  dispersion,  etc.,  work  in  the  opposite  direction. 

*  L.  Pelet-Jolivet:  Roll.  Zeit.,  6,  238-243  (1909). 


CHAPTER  XII 
PROTEIN  BODIES 

PROTEIN  bodies  are  the  most  important  substances  in  plant  and 
animal  tissues.  They  are  thoroughly  discussed  in  works  on  physiological 
chemistry  and  will  not  be  taken  up  here  except  in  their  relation  to  col- 
loidal chemistry.  For  this  purpose  the  properties  of  typical  members 
of  the  more  important  groups  will  be  dealt  with. 

All  proteins  are  produced  in  plants  or  animals.  They  contain  C,  H, 
O,  N,  and  S  in  fairly  constant  proportions  and  are  of  a  most  complicated 
molecular  structure,  in  which  the  amino-acids  play  an  important  part. 
Some  of  them  are  reversible  and  some  irreversible  colloids. 

I.  Simple  Proteids,  or  Albuminous  Bodies.  —  To  this  group  belong 
the  simple,  genuine  proteids  in  the  narrower  sense  of  the  word.    These 
can  be  coagulated  easily.     Albumins  and  globulins  come  under  this 
heading. 

II.  Compound  Proteids.  —  These  are  combinations  of  proteids  with 
other  bodies  and  have  peculiar  properties  due  to  these  latter  called 
"  prosthetischen  Gruppen"  by  Kossel.     To  this  prosthetic  group  be- 
long nucleinic  acid  in  nucleoproteids,  hemitin  in  hemoglobin,  carbo- 
hydrates or  their  derivatives.     These  substances  may  be  separated  by 
the  action  of  acids. 

III.  Albumoids  or  Albuminoids.  —  These  substances  are  not  a  part 
of  the  cell  but  form  a  ground  substance  in  which  the  cells  lie.     They 
have  special  properties  that  justify  a  distinction  being  made  between 
them  and  other  proteids,  although  the  composition  of  the  bodies  into 
which  they  can  be  separated  places  them  with  the  many  proteids. 

Specially  characteristic  is  their  insolubility  in  water,  salt  solution,  and 
in  liquids  from  animal  bodies.  Most  of  them  are  scarcely  soluble  in 
dilute  alkalis. 

Simple  Proteids  or  Albuminous  Bodies. 

These  substances  form  the  chief  constituents  of  the  blood,  the 
muscles  and  glands.  They  are  contained  in  most  of  the  secretions  and 
excretions  of  the  body  but  are  not  present  in  tears,  perspiration,  nor  in 
normal  urine.  They  all  contain  C,  H,  N,  0,  S,  and  some  have  a  small 
amount  of  phosphorus  and  iron.  The  composition  varies  generally 

between  the  following  limits: 

208 


PROTEIN  BODIES  209 

Per  cent 

C 50.6    to  54.4 

H.... 6.5    to    7.3 

N 15       to  17. 6 

S 0.32  to    2.2 

P 0       to    0.85 

O 21.5    to  23. 5 

Simple  proteids  are  odorless,  tasteless,  generally  amorphous  but  some- 
times crystalline.  Their  solutions  are  optically  active  and  rotate  the 
plane  of  polarization  to  the  left.  They  unite  with  either  acids  or  bases 
and  therefore  resemble  amphoteric  electrolytes.  They  are  weak  both 
as  acids  or  bases  and  have  a  large  valence  number.  Otherwise  they 
behave  in  a  manner  similar  to  certain  colloids  such  as  stannic  acid  or 
the  purple  of  Cassius. 

i.   Classification 

A  classification  according  to  constitution  is  not  yet  possible;  it  is 
not  well  enough  understood.  In  physiological  chemistry  a  classifica- 
tion has  gradually  come  into  use  that  gives  a  survey  of  the  field.  The 
classification  of  Cohnheim  *  has  been  chosen,  which  is  also  similar  to 
that  of  Hammarsten,t  Hoppe-Seyler,t  and  Drechsel.§ 

We  are  not  justified  in  considering  the  combination  of  proteids 
with  acids  or  alkalis  as  salts.  Rather  must  we  consider  that  similar 
relations  exist  as  in  the  case  of  the  purple  of  Cassius.  On  the  other 
hand  it  cannot  be  denied  that  the  purely  chemical  point  of  view  has 
much  in  its  favor,  especially  with  regard  to  the  decomposition  products 
which  consist  principally  of  aminoacids,  amphoteric  electrolytes.  In 
this  consideration  the  colloidal  nature  of  the  proteids  must  not  be  lost 
sight  of,  for  ultramicrons  appear  in  quantity  in  the  case  of  the  most 
important  members. 

The  precipitation  of  proteids  by  concentrated  solutions  of  alkalis  is 
generally  reversible.  This  is  the  principle  employed  in  many  methods 
for  separating  them  from  one  another.  Precipitation  with  salts  of  the 
heavy  metals  is  generally  irreversible.  Often  here  irregular  behavior  is 
met  with  that  is  due  to  peptisation  of  the  precipitate  by  the  salts.  A 
survey  of  the  simple  proteids  is  given  in  Table  34. 

*  O.  Cohnheim:  Chemie  der  Eiweisskorper.    Braunschweig  (1904). 

t  O.  Hammersten:  Lehbuch  der  physiologischen  Chemie.    Wiesbaden  (1907). 

J  F.  Hoppe-Seyler:  Handbuch  der  physiologisch-und  pathologisch-chemischen 
Analyse  (7.  Aufl.).  Berlin  (1903). 

§  E.  Drechsel:  Artikel  "Eiweisskorper"  in  Ladenburgs  Handworterbuch  der 
Chemie,  3,  534  (1885). 


210  CHEMISTRY  OF  COLLOIDS 

TABLE  34 
TRUE  PROTEIN  BODIES 

1.  Albumins  (serum,  egg,  lacto-albumins). 

2.  Globulins  (serum,  egg,  cell  globulins). 

a.  Plant  protein  (plant  globulins). 

6.  Fibrinogen,  and  fibrin,  from  the  blood  plasma  of  the  mammals. 
By  fibrin  ferment  these  are  changed  into  fibrin,  and  occasion  the  coagulation 
of  the  blood. 

c.  Myosin  and  myogen,  muscle  protein.      Probably  the  cause  of  rigor 
mortis  is  the  coagulation  of  myosin  under  the  influence  of  a  ferment. 

3.  Phosphorus  containing  proteins  (nucleo-albumin  or  phosphorus  globulin), 
acid  protein,  as  casein,  vitelline  of  the  yolk  of  eggs,  nucleo-albumin  of  the  cell 
protoplasma,  etc. 

4.  a.  Histones,  basic  protein,  precipitated  by  alkalis;    found  in  blood  cor- 
puscles of  geese. 

6.  Protamines,  proteins  free  from  sulfur,  but  rich  in  nitrogen,  in  the  sperma 
of  salmon. 

Albumins.  —  Albumins  are  the  richest  in  sulfur  of  all  proteids,  con- 
taining from  1.6  to  2.2  per  cent  S.  They  are  soluble  in  water  and  are  not 
precipitated  at  ordinary  temperature  by  dilute  solutions  of  alkalis, 
acids  nor  concentrated  solutions  of  NaCl  or  MgS(>4.  Saturated  solutions 
of  (NH^SCU  precipitate  them  completely.  They  are  not  coagulated 
at  boiling  temperatures  unless  considerable  neutral  salts  are  present. 

Globulins.  —  Globulins  are  peculiar  protein  bodies  that  are  solu- 
ble in  dilute  solutions  of  neutral  salts,  but  not  in  pure  water.  Dialysis, 
filtration  through  Bechhold's  filter,  or  great  dilution  precipitates  them. 
They  are  again  dissolved  by  traces  of  alkali,  while  neutralization  of 
the  solutions  separates  them  out  again.  Concentrated  solutions  of 
NaCl  or  MgS04  precipitate  them  partially  or  completely.  Half-satu- 
rated solutions  of  (NHj^SCU  also  precipitate  them. 

The  separation  of  proteids  is  made  easier  by  the  fact  that  specific 
liquids  of  the  body  contain  only  certain  of  the  proteids.  Thus  egg 
albumin  contains  globulin,  albumin  and  ovomucoid;  blood  plasma 
contains  fibrinogen,  globulin  and  albumin.  As  an  example  of  how  the 
separations  are  carried  out  the  isolation  of  certain  members  from  the 
blood  plasma  may  be  cited. 

Horse  blood  is  prevented  from  coagulating  by  the  addition  of  one- 
tenth  volume  of  1  per  cent  (NH4)2C204  and  the  blood  corpuscles  are  al- 
lowed to  fall  as  sediment  to  the  bottom.  Next  3  parts  of  a  saturated 
solution  of  (NH4)2S04  to  7  parts  of  plasma  are  added,  whereby  fibrinogen 
is  precipitated.  The  solution  contains  globulin  and  albumin.  Four 
parts  of  saturated  (NH4)2S04  solution  are  now  added  to  10  parts  of 
the  solution  and  globulin  separates  out.  The  albumin  can  be  pre- 
cipitated by  saturating  the  filtrate  with  solid  (NH^SO^  The  differ- 
ent fractions  may  be  purified  by  washing  with  the  corresponding  salt 
solutions  and  then  dialyzing.  If  acid  is  carefully  added  with  the 


PROTEIN  BODIES  211 

(NH4)2S04  until  the  solution  begins  to  be  turbid  the  albumin  may  be 
obtained  in  a  crystalline  form. 

The  separation  of  proteids  in  the  white  of  eggs  is  achieved  by  a  similar 
process.  (NH^SC^  is  added  until  the  solution  is  one-half  saturated 
in  order  to  precipitate  the  globulin.  The  filtrate  contains  albumin 
and  ovomucoid.  A  part  of  the  former  may  be  obtained  in  crystalline 
form  by  adding  acid  until  the  liquid  becomes  turbid.  After  standing 
24  hours  microscopic  crystals  of  egg  albumin  are  obtained  that  may  be 
further  purified  by  recrystallization.  The  state  of  purification  can  be 
best  determined  by  means  of  the  gold  number,  page  106.  Further 
additions  of  (NH^SC^  to  the  filtrate  precipitate  more  albumin  in  amor- 
phous form.  These  fractions  do  not  appear  to  be  amorphous  modifi- 
cation of  the  crystalline  albumin,  but  differ  from  the  latter  in  both 
chemical  and  physical  properties. 

It  should  be  noted  that  globulins  are  by  no  means  simple  uniform 
substances.  By  suitable  reactions  they  also  may  be  fractionated  into 
different  bodies. 

Compound  Proteids  and  Albuminoids 
SURVEY  OF  COMPOUND  PROTEIDS  AND  ALBUMINOIDS 

a.   Compounds  of  proteids  with  bodies  not  proteids. 

1.  Nucleoproteids;  proteids  with  nucleinic  acid. 

2.  Hemoglobins  and  related  substances;  globins  with  Hematin. 

3.  Glycoproteids  (Mucines,  Mucoids;  compounds  of  proteids 

with  carbohydrates). 

Nucleoproteids  are  constituents  of  the  cell  nuclei  and  may  be  sepa- 
rated with  proteids  and  nuclein,  or  proteids  and  nucleinic  acid.  The 
latter  may  be  separated  into  pyramidins  and  purine  derivatives  among 
other  substances.  Hemoglobins  will  be  taken  up  later.  To  the  glyco- 
proteids  belong,  among  other  bodies,  ovomucoid  that  is  not  easily 
coagulated. 

6.  Albuminoids. 

1.  Keratin  (horny  substances  of  the  human  and  other  animal 

bodies) . 

2.  Kollagen. 

3.  Elastins  (constituents  of  elastic  tissues). 

4.  Fibroin  (contained  in  the  fibers  of  silkworm). 

5.  Spongin  (found  in  the  structure  of  the  common  sponge), 

Amyloid,  Albuminoid. 

As  already  stated  the  albuminoids  form  the  structure  tissues  of 
animals  and  differ  from  ordinary  proteids  in  their  great  stability  toward 


212  CHEMISTRY  OF  COLLOIDS 

solvents  and  chemical  influences.  Keratin  is  the  chief  constituent 
of  the  outlayer  of  the  epidermis,  hair,  feathers,  horns,  hoofs,  and 
nails. 

Kollagen.  —  The  fundamental  substance  of  the  bones  and  carti- 
lages. By  long  boiling  in  water,  faster  in  dilute  HC1,  kollagen  goes 
into  glutine,  glue,  or  gelatine. 

Decomposition  Products  of  Protein  Bodies 

SURVEY 

1.  Acid  Albumins  and  Alkali  Albuminates. 

By  treating  proteids  with  alkalis  or  acids,  decomposition  products 
are  obtained  that  are  closely  related  to  the  proteins.  They  are  called 
alkali  albuminates  and  acid  albumins,  or  syntonins.  Paal  *  obtained 
his  protalbinic  and  lysalbinic  acids  by  treatment  of  proteids  with 
alkalis.  The  former  are  closely  allied  to  alkali  albuminates  and  the 
latter  to  the  albumoses.  The  alkali  albuminates  are  excellent  pro- 
tective colloids;  so  are  protalbinic  and  lysalbinic  acids.  The  gold 
number  of  the  sodium  salt  of  protalbinic  acid  is  0.03  to  0.08  while 
that  of  lysalbinic  acid  is  0.02  to  0.06.  Concerning  their  use  in  the 
preparation  of  colloidal  metals  see  Chapter  V. 

a.  Pepsin  Digestion.  —  Albumoses  and  peptones  are  generated  by 
the  pepsine  digestion  of  proteids.  Stomach  juices  contain  enzymes 
together  with  mucous  substances,  salts  and  HC1  (0.4  per  cent).  Pep- 
sin is  one  of  the  most  important  of  these  enzymes  and  is  active  for 
digestive  purposes  only  in  the  presence  of  HC1.  Not  only  natural 
proteids  undergo  digestion  but  also  denatured  proteids  such  as  coagu- 
lated proteid.  Distention  occurs  first  and  then  dissolution.  The 
maximum  temperature  at  which  pepsin  digestion  takes  place  is  slightly 
higher  than  the  temperature  of  the  body,  viz.,  at  40°  C.  Syntonins 
and  primary  albumoses  are  first  formed;  then  secondary  albumoses 
and  finally  peptones,  the  end  product  of  pepsin  digestion. 

Albumoses  diffuse  slowly  in  aqueous  solutions,  not  at  all  or  only 
partially  through  membranes,  and  Bechhold's  filter  retains  them.  Satu- 
rated (NH4)2S04  precipitates  them.  Peptones,  on  the  other  hand,  re- 
semble the  crystalloids  much  more.  They  diffuse  fairly  easily,  pass 
through  membranes  slowly,  and  are  not  precipitated  by  (NH^SO^ 
In  contradistinction  to  peptones,  albumoses  and  peptones  do  not  coag- 
ulate on  boiling. 

Albumoses  and  peptones  may  be  separated  by  the  following 
scheme : 

*  C.  Paal:  Ber.,  36,  2195-2206  (1902). 


PROTEIN  BODIES  213 

50  QC.  albumose  solution 
+  50  cc.  saturated  (NH4)2S04. 


I  I 

Ppt.  I  (Primary  albumoses)  Filtrate  I 

Dissolved  in  H20  and  dialyzed  Add  solid  (HN4)2SO4 


Ppt.  II  Filtrate  Ppt.  secondary        Filtrate 

Heteroalbumoses  Protalbumoses  Albumoses  Peptones 

The  products  of  pepsin  digestion  were  previously  called  peptones 
regardless  of  whether  they  contained  albumoses  or  not.  The  larger 
part  of  Witte-peptone  procurable  on  the  market  consists  of  albumoses. 
According  to  Bechhold  it  is  possible  to  separate  albumoses  from  one 
another  by  filtration.  The  glacial  acetic  acid  collodion  filter  retains 
the  primary  albumoses,  while  the  deuteroalbumoses  pass  through. 
These  may  be  filtered  out  by  a  8  to  10  per  cent  filter.  Albumoses  are 
frequently  called  proteoses  at  the  present  time. 

Heteroalbumoses,  obtained  by  dialysis  in  the  above  scheme  and 
therefore  behaving  similarly  to  globulin,  have  a  high  gold  number 
(according  to  Zunz,  0.01  to  0.075).  Protalbumoses  are  much  less 
efficacious  as  protective  colloids;  gold  number  1.6  to  3.4.  Secondary 
albumoses  and  the  majority  of  the  peptones  have  the  property  of 
turning  red  colloidal  gold  solutions  blue  without  the  help  of  other 
electrolytes,  see  Chapter  V.  Neither  albumoses  nor  peptones  are 
simple  substances.  By  alcohol  they  may  be  further  fractionated,  but 
even  these  products  are  also  mixtures. 

6.  Pancreas  Digestion.  —  Trypsin,  a  ferment  found  in  the  intestines, 
causes  a  considerable  splitting  of  the  proteid  products  to  aminoacids 
(leucin  tyrosin,  asparaginic  acid,  try ptophanes,  etc.).  Pancreas  diges- 
tion takes  place  in  alkali  reaction  and  changes  deuteroalbumoses  to 
anti-  and  hemi-peptone;  the  latter  is  split  into  aminoacids. 

The  chemistry  of  proteins  has  received  a  very  decided  advance  by  the 
excellent  work  of  Emil  Fischer  and  his  collaborators  through  the  synthesis 
of  polypeptides  from  amino-acids.  For  a  detailed  account  of  the  poly- 
peptides  reference  should  be  made  to  works  on  physiological  chemistry. 

2.  General  Behavior  of  Protein  Bodies 
OSMOTIC  PRESSURE 

Careful  osmotic  pressure  investigations  on  protein  bodies  have  been 
made  by  Hiifner  and  Gausser  (see  hemoglobin,  page  231),  Moore  and 
his  collaborators,*  and  by  Lillie.f  Moore  and  Roaf  have  shown  that 

*  B.  Moore  and  W.  H.  Parker:  Amer.  Journ.  of  Physiol.,  7,  261  (1902).  The 
same  with  H.  E.  Roaf:  Biochemical  Journ.,  2,  34  (1906). 

t  R.  S.  LilUe:  Amer.  Journ.  of  Physiol.,  20,  127-169  (1907). 


214 


CHEMISTRY  OF  COLLOIDS 


the  osmotic  pressure  of  gelatin  solutions  increases  more  rapidly  with 
the  temperature  than  would  be  predicted  from  the  laws  of  solutions. 
This  agrees  with  the  tendency  of  gelatins  to  form  larger  complexes  and 
finally  to  harden  as  the  temperature  is  lowered.  These  authors  have 
also  observed  interesting  hysteresis  phenomena.  (See  gelatins,  page 
223.)  Lillie  investigated  the  osmotic  pressure  of  egg  albumin,  and  also 
the  effect  of  the  addition  of  crystalloids  to  gelatin  solutions.  The 
proteins  were  put  in  the  collodion  sack  of  the  osmometer  (page  35) 
and  the  pressure  against  pure  water  measured.  The  maximum  height 
of  the  column  was  reached  in  about  20  hours,  and  remained  constant 
for  several  hours.  The  influence  of  the  crystalloids  was  determined 
by  adding  amounts  to  both  the  inner  and  outer  liquids  until  the  con- 
centration in  each  was  equal.  In  doing  this  great  care  must  be  taken 
with  regard  to  stirring,  etc.,  in  order  to  avoid  irregular  results. 

TABLE  35 


Egg  albumin  at  room  temperature. 

Pressure  in 
mm.  hg. 

1  .  25  per  cent  egg  albumin  
-f-  ^  mol   cane  sugar 

22.4 
21.5 

-f-  ^  mol   dextrose 

21.8 

Nonelectrolytes  show  little  influence  on  the  osmotic  pressure  of 
protein  or  gelatin  solutions.  Cane  sugar,  dextrose,  glycerine,  urea, 
each  to  the  amount  of  1.25  per  cent,  were  added  to  a  gelatin  solution. 
The  column  stood  at  6.2  mm.  without  the  addition.  Urea  raised  it  to 
7.3,  while  glycerine  and  dextrose  gave  5.8  and  5.9  respectively.  The 
effect  of  the  addition  on  egg  albumin  can  be  seen  in  Table  35. 

TABLE  36 


Gelatin  1 

5  per  cent. 

Without  HC1. 

8.2  mm.  Hg. 

Without  KOH. 

6.2  mm.  Hg. 

AT 

60 

AT 

«   1 

3100 

N 

.0 
190 

3100 
N 

97  4. 

2050 
N 
1550 
N 
770 
N 

17.9 
32.4 
39  3 

1240 
2V 
620 
N 
310 

33.1 
33.2 

422 

PROTEIN  BODIES 


215 


On  the  contrary  the  influence  of  electrolytes  is  considerable.  The 
presence  of  acids  and  alkalis  causes  the  ultramicrons  of  gelatin  to  sub- 
divide, and  the  column  to  rise  in  consequence  as  seen  in  Table  36. 

In  the  case  of  egg  albumin  the  acid  (HC1)  caused  a  fall  at  first  and 

then  a  rise. 

TABLE  37 


HCl. 

Pressure  in 
mm.  Hg. 

HCl. 

Pressure  in 
mm.  Hg. 

D 

N 
3100 
N 
1240 

25.6 
20.7 

11.5 

N 
620 
N 
310 

20.4 
22.2 

The  effect  of  HCl  on  gelatin  can  be  explained  only  on  the  ground 
of  a  subdivision  of  the  ultramicrons  and  simultaneous  or  subsequent 
ionization.  The  subdivision  is  rendered  more  probable  by  the  fact 
that  submicrons  are  visible  in  gelatins  at  ordinary  temperatures. 
This  behavior  of  gelatin  with  HCl  agrees  with  experiments  of  Wo. 
Ostwald  *  who  found  that  small  amounts  of  HCl  decrease  the  disten- 
tion  while  large  amounts  increase  it  very  greatly.  Alkali  also  increases 
the  distention. 

One  might  assume  that  acids  and  alkalis  cause  the  decomposition 
of  the  gelatin  molecule  into  albumoses  and  peptones  and  therefore  the 
pressure  must  increase.  Against  this  assumption  is  the  experimental 
fact  of  the  small  concentration  of  the  HCl;  and  secondly  that  the 
original  pressure  is  almost  entirely  restored  after  the  removal  of  the 
HCl  by  dialysis.  Neutral  salts,  contrary  to  the  effect  of  acids,  lower 
the  osmotic  pressure  of  both  gelatin  and  albumin  solutions.  This  is  in 
accordance  with  the  fact  that  neutral  salts  have  a  coagulating  effect  on 
protein  solutions  long  before  the  precipitating  concentration  is  reached. 
The  number  of  particles  is  lessened  and  therefore  the  osmotic  pressure 
decreased.  The  influence  is  apparent  from  the  following  table. 

TABLE  38 


NaCl. 

RD. 

NaCl. 

RD. 

n 
12 

0.18 

n 

48 

0.26 

n 

0  23 

n 

24 

96 

0.37 

RD  = 


Osmotic  pressure  of  the  salt  solution 
Osmotic  pressure  of  the  original  solution 
*  Wo.  Ostwald:  Pflugers  Archiv.  f.  d.  ges,  Physiol.,  108,  563-589  (1905). 


216  CHEMISTRY  OF  COLLOIDS 

Several  hundred  experiments  were  carried  out  to  determine  the  re- 
lation between  the  fall  in  pressure  and  the  nature  of  the  electrolyte. 
It  turned  out  that  both  the  nature  of  the  anion  and  that  of  the  cathion 
are  factors.  Hofmeister  *  and  Pauli  f  have  arranged  several  anions 
in  the  descending  order  of  their  reducing  effect  on  the  osmotic  pressure 
of  protein  solutions.  S04  >  Cl  >  NO3  >  Br  >  I  >  CNS.  Both  series 
of  experiments  point  to  the  conclusion  that  the  reduced  pressure  is 
due  to  partial  coagulation  of  the  particles. 

Pure  protein  solutions  lose  very  little  of  their  osmotic  pressure  through 
violent  shaking.  If  electrolytes  are  present  the  pressure  is  reduced 
from  10  to  20  per  cent. 

Lillie  J  has  found  that  the  increase  of  osmotic  pressure  with  the  con- 
centration is  not  quite  proportional,  the  pressure  increasing  faster 
than  the  concentration.  This  result  is  in  agreement  with  the  fact  that 
many  proteids,  such  as  globulin,  are  precipitated  by  diluting,  whereby 
the  number  of  particles  is  lessened  and  therefore  the  pressure  lowered 
beyond  the  normal  amount.  Duclaux  obtained  similar  results  with 
colloidal  iron  oxide  solutions,  but  the  explanation  is  doubtless  different 
from  that  in  the  case  of  proteids.  See  page  167. 

Precipitation  of  Proteids  with  Electrolytes 

The  precipitation  of  proteids  by  electrolytes  has  been  especially 
well  investigated  by  Hofmeister  §  and  his  students.  It  is  shown  that 
the  precipitation  of  egg  or  serum  albumin  by  alkali  or  magnesium  salt 
mixtures  is  reversible;  that  is  the  precipitates  will  dissolve  in  water  on 
the  removal  of  the  salt.  Large  amounts  of  the  salt  are  necessary  for 
precipitation.  Salts  of  the  alkaline  earth  metals  in  large  amounts  also 
cause  precipitation,  but  the  precipitate  rapidly  becomes  insoluble. 
Lower  concentrations  cause  a  reversible  precipitation  and  the  precipi- 
tate is  redissolved  by  concentrations  somewhat  greater. 

In  connection  with  the  precipitation  by  salts  of  the  alkali  metals 
it  was  found  that  the  anions  had  a  distinct  effect.  With  the  same 
cathion  the  tartrate  had  a  greater  effect  than  the  sulfate,  and  so  on. 
The  series  is  given  below: 

Citrate  >  Tartrate  >  Sulfate  >  Acetate  >  Chloride  >  Nitrate  > 
Iodide  >  Sulfocyanate. 

*  F.  Hofmeister:  Archiv.  f.  experim.  Pathol.  u.  Pharmakol.,  24,  1-30  (1889). 

t  W.  Pauli:  Hofmeisters  Beitrage  z.  chem.  Physiol.  u.  Pathol.  3,  225-246 
(1902). 

t  Lillie:  I  c. 

§  S.  Lewith:  Archiv.  f.  experim.  Pathol.  u.  Pharmakol.,  24,  1-16  (1888).  F. 
Hofmeister:  Ibid.,  24,  247-260;  25,  1-30  (1889). 


PROTEIN  BODIES  217 

It  will  be  seen  that  this  series  agrees  with  that  of  Lillie  given  on  page 
216.  The  iodide  and  sulfocyanate  do  not  cause  coagulation  at  all. 
Posternak  *  and  also  Pauli  f  have  found  that  the  series  is  reversed  if 
the  experiments  are  carried  out  in  slightly  acid  solution.  There  iodides 
and  sulfocyanates  have  the  greatest  precipitating  effect  while  citrates 
have  the  least.  In  strongly  acid  solutions  the  precipitation  is  not  re- 
versible; that  is  the  precipitate  will  not  dissolve  in  pure  water.  In 
dilute  alkali  solutions  the  series  has  the  same  order  that  it  has  in 
neutral  solutions.  In  both  cases  the  particles  are  charged  negatively, 
but  are  charged  positively  in  acid  solution. 

Pauli  t  found  that  there  was  a  relation  between  the  precipitating 
effect  on  protein  bodies  and  the  physiological  effect.  Citrates,  tar- 
trates,  and  sulfates  excite  the  intestines  and  raise  the  blood  pressure, 
while  nitrates,  bromides,  and  sulfocyanates  lower  the  blood  pressure. 
Iodides  and  sulfocyanates  cause  side  reactions  that  result  in  colds  in 
the  head  and  in  acne 

Neutral  Proteids 

Pauli  §  has  succeeded  in  taking  a  further  step  in  this  field  by  pre- 
paring a  proteid  hydrosol  almost  free  from  electrolytes.  This  hydrosol, 
obtained  by  dialyzing  for  8  days  has  somewhat  different  properties 
from  the  ordinary  proteid  solution.  It  coagulates  completely  on 
boiling  without  the  presence  of  electrolytes,  and  suffers  irreversible 
precipitation  by  alcohol.  It  does  not  migrate  in  the  electric  current, 
or  at  least  very  little.  Pauli  has  called  this  preparation  neutral  or 
amorphous  proteid.  He  determined  by  migration  experiments  that 
acids  charge  it  positively  and  alkalis  negatively.  Hardy  If  had  pre- 
viously recorded  the  same  effects  of  acids  and  alkalis  on  proteid  solu- 
tions. The  influence  of  acids  and  alkalis  on  the  direction  of  the 
migration  is  therefore  independent  of  the  size  of  the  particles. 

A  series  of  phenomena  are  in  agreement  with  the  electrical  charge, 
or  the  ionization  of  these  amphoteric  proteids.  The  viscosity  in- 

79 

creases  with  small  additions  of  HC1,  T      HC1  causing  a  change  in  the 


inner  friction  of  1068  and  1209.  Pauli  attributes  this  to  hydration  of 
the  ultramicrons  because  of  their  ionization.  Other  properties  also 
change,  for  while  amorphous  proteid  can  be  precipitated  with  alcohol  a 
slight  amount  of  acid  or  alkali  prevents  this.  Larger  amounts  of  acids 

*  S.  Posternak:  Annales  de  1'institut  Pasteur,  15,  85  ff.  (1901). 

t  W.  Pauli:  Hofmeisters  Beitrage  z.  chem.  Physiol.  u.  Pathol.,  5,  27-55  (1903). 

t  W.  Pauli:  Wiener  klin.  Wochenschr,  Nt.  20  (1904). 

§  W.  Pauli:  Beitrage  z.  chem.  Physiol.  u.  Pathol.,  7,  531-547  (1906). 

U  W.  B.  Hardy:  Zeit.  f.  phys.  Chemie,  33,  385-400  (1900). 


218  CHEMISTRY  OF  COLLOIDS 

restore  the  normal  precipitation,  probably  because  of  the  driving  back 
of  the  dissociation.  As  explained  on  page  219,  NaCl  and  NaNO3  and 
other  neutral  salts  have  a  similar  effect.  Conductivity  measure- 
ments have  shown  that  the  charge  on  the  particles  was  reduced  by  these 
salts. 

Electric  Charge  on  the  Proteid  Particles 

The  particles  of  neutral  proteid  of  Pauli,  boiled  hydrosols,  or  sus- 
pensions of  proteids  (but  not  coagulated  suspensions  with  submicro- 
scopic  particles)  are  charged  positively  by  acids  and  negatively  by 
alkalis.  In  general  the  charge  may  be  caused  by  the  adsorption  of 
the  hydrogen  or  hydroxyl  ion  according  to  the  following  scheme: 

[U]  +H+^±[F|H+. 
[U]  +  OH=  ^  [u]  OH". 

In  most  cases  it  is  more  probable  that  the  acid  or  alkali  forms  a 
chemical  combination  with  some  of  the  surface  molecules  and  these  give 
forth  ions.  The  remaining  ultramicron  would  therefore  be  charged. 

It  is  known  that  a  neutral  salt  may  be  adsorbed  without  affecting 
the  direction  of  migration. 


This  adsorption,  or  possibly  chemical  combination,  may  influence  the 
solubility  and  other  chemical  properties,  while  it  does  not  affect  the 
charge. 

Proteids,  containing  as  they  do  both  amido-  and  carboxyl  groups, 
resemble  amino-acids  (out  of  which  they  are  partly  made)  to  a  large 
degree  in  their  chemical  behavior.  It  is  well  therefore  to  consider 
some  of  their  chemical  reactions  as  Hardy  *  and  Pauli  f  have  done. 
According  to  Pauli  the  presence  of  the  amido-  and  carboxyl  groups  may 
be  represented  thus: 


R( 
NCOOH 

Every  molecule  contains  several  amido  and  hydroxyl  groups  and 
therefore  every  ultramicroscopic  complex  must  contain  many  more. 
For  the  sake  of  simplicity  let  us  consider  only  one  of  these  groups. 

If  an  amine,  R  •  NH2,  and  HC1  come  into  contact  an  ammonium  salt 
will  be  formed.  R  .  NH2  +  HC1  ^  RNH3C1. 

This  salt  is  dissociated  in  solution: 

RNH3C1  ?±  RNH3+  +  01". 

*  W.  B.  Hardy:  Journ.  of  Physiol.,  33,  251-337  (1905-1906). 

t  W.  Pauli  und  H.  Handovsky:  Biochem.  Zeit.  18,  340-371  (1909). 


PROTEIN  BODIES  219 

Similarly  an  amido  group  and  HC1  would  form  a  salt: 
XNH2  XNH3C1 


+  cr. 

COOR  COOR  COOR 

Or  simply: 

.NH2  .NH3+ 

R/         +H+^±R( 
XCOOH  XCOOH 

Vice  versa  a  neutral  proteid  is  charged  negatively  by  NaOH. 

.NH2  NH2 

H20. 

It  makes  little  difference  in  the  point  of  view  whether  we  consider 

the  radicle  R  .    ^^^TT  on  a  single  molecule  or  on  an  ultramicron.     In 
> 


the  latter  case  our  equation  would  be  written 

/NH2  NH2 

~ 


/ 

R  (  +  OH~^±  UlR  ;  +  H2O. 

XCOOH  N         - 


That  the  acids  are  really  bound  to  the  proteids  has  been  demon- 
strated by  Sjoquist,*  Cohnheim,t  and  others.  Bugarszky  and  Lieber- 
mann  |  showed  that  at  higher  concentrations  both  the  H+  ion  and 
Cl~  ion  were  bound,  also  similarly  NaOH.  Barratt§  has  shown  that 
both  H+  and  OH~  are  bound  by  living  protoplasm. 

As  already  stated  the  electrically  charged  particles  are  to  be  regarded 
as  the  cause  of  the  increased  viscosity  in  protein  solutions  (Laqueur  and 
Sackur,1T  Hardy,  Pauli,  and  others)  and  Pauli  assumes  the  greater 
hydration  of  the  proteid  molecules,  or  ion  molecules.  The  union  of 
the  'water  with  the  proteid  is  not  necessarily  a  chemical  one  in  the 
strict  sense  of  the  word.  The  hydrate  formation  corresponds  to  the 
stability  of  the  hydrosol  toward  alcohol.  If  the  charge  is  neutralized 
the  proteid  gives  up  the  water  and  becomes  precipitable  by  alcohol. 

As  stated  on  page  218  large  concentrations  of  HC1  precipitate  the 
proteid  ionized  by  small  amounts  of  this  acid.  This  may  be  explained 
on  the  ground  that  ionization  of  the  acid  drives  back  the  ionization  of 
the  proteid  complex,  and  therefore  it  is  less  stable.  NaCl  would  have 

*  J.  Sjoquist:  Skand.  Archiv.  f.  Physiol.,  6,  277-376  (1895). 

t  O  Cohnheim:  Zeit.  f.  Biol.,  33,  489-520  (1896).  Ders.  und  H.  Krieger:  Ibid., 
40,  95-116  (1900). 

J  S.  Bugarszky  und  L.  Liebermann:  Archiv.  f.  d.  ges.  Physiol.,  72,  51-74  (1898). 

§  J.  O.  Wakelin  Barratt:  Zeit.  f.  allg.  Physiol.,  6,  10-33  (1905). 

If  E.  Laqueur  und  O..Sackur:  Hofmeisters  Beitrage  z.  chem.  Physiol.  u.  Pathol., 
3,  193-224  (1903). 


220  CHEMISTRY  OF  COLLOIDS 

a  similar  effect,  or  in  fact  any  neutral  salt.  In  the  latter  case,  accord- 
ing to  Hardy  and  also  Pauli,  hydrogen  ion  will  be  set  free.  In  other 
words  the  metal  ion  will  be  substituted  for  the  hydrogen  ion  in  the 
complex  as  shown  in  the  following  schemes  according  to  Hardy: 

H  Na 

NH, 


R  C\+  NaN03  -*  R  C\  +  HN03. 

XCOOH  NCOOH 

Pauli  holds  the  following  reaction  to  be  the  more  probable: 

H  H 

NH2  NH»( 

XC1  +  HN03 


XCOOH  NCOONa 

Precipitation  with  the  Salts  of  Heavy  Metals 

Precipitation  with  this  class  of  salts  differs  from  that  with  the  alkali 
salts  in  that  small  amounts  of  the  former  are  sufficient,  and  that  the 
precipitate  is  insoluble  in  water.  The  process  recalls  the  irreversible 
precipitation  of  colloids  where  likewise  a  small  amount  of  the  salt 
caused  the  colloid  to  become  insoluble  in  water.  Without  going  into 
the  discussion  of  a  possible  chemical  combination  the  process  may  be 
represented  thus: 

~~  E  Zn. 


In  case  that  the  charge  on  the  proteid  complex  was  due  to  the  dis- 
sociation of  hydrogen  ion,  the  neutralization  by  the  metal  ion  would 
not  set  any  more  acid  free. 

The  precipitate  is  sometimes  soluble  in  an  excess  of  the  precipitating 
agent.  This  solubility  may  be  caused  by  peptisation,  or  may  be  con- 
sidered in  the  same  light  as  the  formation  of  complex  ions.  Szillard  * 
has  assumed  peptisation  to  be  the  cause,  for  he  succeeded  in  peptising 
coagulated  proteids  with  the  salts  of  heavy  metals,  such  as  uranylni- 
trate,  thorium  nitrate,  zinc  and  copper  sulfates,  although  the  two  latter 
required  a  large  quantity.  To  explain  this  from  the  colloidal  chemical 
standpoint  it  is  but  necessary  to  assume  that  the  coagulated  particles 
take  more  zinc  ion  and  become  charged  again,  but  this  time  positively. 
The  equation  would  be  as  follows: 


The  peptised  colloid  is  positively  charged.    As  in  the  case  of  zinc  sul- 
fate  very  often  a  still  larger  excess  will  cause  a  second  precipitation. 

*  B.  Szillard:  Journ.  de  chim.  phys.,  6,  495-496  (1907). 


PROTEIN  BODIES  221 

Frequently  the  solubility  of  the  salt  in  question  is  not  great  enough  to 
cause  the  second  precipitation;  i.e.,  copper  sulfate.  The  precipitation 
of  proteids  by  zinc  sulfate  recalls  the  irregular  series  of  Bechhold,* 
Neisser  and  Friedemann.f  Silver  nitrate  and  many  other  salts  of  the 
heavy  metals  cause  irreversible  precipitation  of  proteids,  and  the 
precipitate  is  not  soluble  in  an  excess  of  the  reagent. 

Coagula  ion  and  Denaturization  of  Proteids 

When  proteids  are  raised  to  the  boiling  point  they  suffer  specific 
changes  of  two  kinds :  they  either  separate  out  as  a  curd  in  the  normal 
way,  or  they  remain  dissolved  as  a  colloid  with  other  properties.  The 
visible  change  during  the  formation  of  the  curd  or  precipitate  is  favored 
in  a  large  degree  by  the  presence  of  salts  and  traces  of  acid  (Koch 
test  for  proteids.)  Moderately  dialyzed  proteids  do  not  coagulate  on 
boiling,  and  no  visible  action  takes  place  with  traces  of  alkalis  and 
acids.  Boiled  solutions  of  proteids  relatively  free  from  electrolytes 
can  be  brought  to  visible  coagulation  immediately  by  the  addition  of 
neutral  salts.  These  differ  from  the  original  solutions  in  that  they  are 
filled  with  innumerable  submicroscopical  particles.  In  general  be- 
havior they  resemble  the  irreversible  colloids.  If  such  a  solution  is 
made  alkaline  the  particles  are  charged  negatively  and  the  valence  of 
the  cathion  is  a  factor  in  the  precipitation.J  On  the  contrary  the 
anion  is  the  important  factor  if  the  colloid  has  been  charged  positively 
by  the  addition  of  acid. 

Pauli  and  Handovsky§  have  observed  an  interesting  case  of  de- 
naturization.  Small  quantities  of  salts  of  the  alkali  and  alkaline 
earth  metals  raise  the  coagulation  temperature  of  well-dialyzed  proteid 
solutions;  in  other  words  tend  to  prevent  coagulation  by  heat.  If 
higher  concentrations  of  salts  are  employed  the  alkali  salts  may  be 
divided  into  three  groups  that  differ  in  their  effect  on  the  coagulation 
temperature.  One  group  is  comprised  of  iodides  and  sulfocyanates, 
which  in  higher  concentration  completely  prevent  coagulation.  This 
is  interesting  and  it  was  a  question  whether  the  iodide  prevents  the 
denaturization  of  the  proteid  by  heat,  or  prevents  the  precipitation  oi 
the  denatured  proteid.  Experiment  showed  that  the  latter  was  the 
case,  and  that  the  heat  actually  causes  the  denaturization.  By  dialysis 
the  excess  of  salt  was  removed  and  coagulation  occurred  immediately. 

*  H.  Bechhold:  Zeit.  f.  phys.  Chemie,  48,  385-423  (1904). 

t  M.  Neisser  und  U.  Friedmann:  Munch,  med.  Wochenschr.,  61,  465-469,  827- 
831  (1904). 

$    Hardy:  I  c. 

§  W.  Pauli  und  H.  Handovsky:  Hofmeisters  Beitrage  z.  chem.  Physiol.  u.  Pathol., 
5,  27-55  (1903). 


222  CHEMISTRY  OF  COLLOIDS 

Proteids  may  also  be  denatured  by  an  extended  treatment-  with 
alcohol  and  Pauli*  has  found  that  even  salts  and  acid  will  work  the 
change  in  time.  Alcohol  at  first  precipitates  a  soluble  variety  of  proteid, 
which  becomes  insoluble  on  long  standing. 

Behavior  of  Globulins 

As  already  pointed  out  globulins  have  the  property  of  being  soluble 
in  dilute  salt  solutions  but  not  in  pure  water.  Excess  of  salt  precipi- 
tates them,  but  they  will  redissolve  when  the  dilution  is  increased  suf- 
ficiently. These  peculiarities  are  due  to  the  chemical  composition  of 
the  substances.  Hardy,  f  who  has  carefully  studied  the  solubility  re- 
lations, regards  globulin  as  a  substance  insoluble  in  water,  but  which 
unites  with  alkali  salts  to  form  soluble  complex  salts,  the  reaction  being 
similar  to  the  dissolution  of  silver  chloride  by  ammonia.  He  believed 
that  globulin  and  NaCl  formed  a  complex,  NaGlob  .  Cl,  the  amount 
naturally  depending  upon  the  concentration  of  the  NaCl.  This  complex 
could  dissociate  thus: 

CIGlob.  Na  ^  CIGlob."  +  Na+. 

The  dissociation  would  be  dependent  upon  the  concentration  of  the 
NaCl,  and  at  great  dilution  the  complex  ion  would  be  completely  dis- 
sociated and  the  globulin  would  therefore  be  insoluble. 

CIGlob.  Na  =  Na+  +  Cl~  +  Globulin  (insoluble). 
At  moderate  concentrations  of  the  NaCl  a  part  of  the  proteid  is  dissolved 
in  the  form  of  an  electrolyte,  while  the  remainder  is  present  as  sub- 
microns.  At  high  concentrations  of  the  salt  the  dissociation  is  so  far 
driven  back  that  precipitation  occurs.  The  experimental  results  of 
Michaelis  {  with  the  ultramicroscope  are  in  accord  with  this  point  of 
view.  He  observed  that  globulin  solutions  diluted  with  water  con- 
tain a  much  larger  number  of  submicrons  than  those  at  the  same  dilu- 
tion when  salt  was  present.  However  the  colloidal  solutions  are  stable 
which  is  most  easily  explained  on  the  assumption  that  the  ultramicrons 
have  adsorbed  globulin-chloride  ion  according  to  some  such  reaction 
as  the  following: 

[0]  + CIGlob.  ~    .<=>  |_UJ  ClGlob.~ 

Hardy  §  has  also  investigated  the  union  of  globulin  with  acids  and 
bases.  He  assumed  the  formation  of  globulin  ion  that  would  be 

*  W.  Pauli  und  H.  Handovsky:  Hofmeisters  Beitrage  z.  chem.  Physiol.  u.  Pathol., 
6,27-55(1903). 

t  W.  B.  Hardy:  Journ.  of  Physiol.,  33,  251-337  (1905);  Proc.  Roy.  Soc.,  79 
(B.),  413-426  (1907). 

$  L.  Michaelis:  Virchows  Archiv.  f.  pathol.  Anat.  u.  Physiol.,  179,  195-208  (1905). 

§  Hardy:  L  c. 


PROTEIN  BODIES  223 

affected  by  hydrolysis.  Naturally  the  globulin  salts  of-  weak  acids 
would  hydrolyze  to  a  greater  degree  than  those  of  strong  acids,  and 
therefore  the  formation  of  ultramicrons  would  be  favored  in  the  former 
case. 

Ultramicrons  formed  from  globulin  salts  are  doubtless  charged  by 
the  adsorption  of  ions  in  a  manner  similar  to  that  just  indicated.  It 
is  interesting  that  the  charged  ultramicrons  have  a  greater  mobility 
under  the  influence  of  a  potential  fall  than  the  ordinary  proteid  ion. 
Hardy  discriminates  between  true  ions  and  pseudo-ions.  The  latter 
are  charged  ultramicrons. 

3.  Special  Examples 

A.  Gelatins,  Their  Preparation,  and  Properties 

Colorless  purified  glue  or  glutin  is  known  as  gelatin.  Glue  is  made 
by  boiling  collagen  in  water  to  which  acid  has  been  added.  The  chemi- 
cal composition  is  approximately  as  follows: 

Per  cent. 

C 49      to  50 

H 6. 4  to    6.7 

N 17. 8  to  17. 9 

S 0.2  to    0.7 

0 24      to  25 

The  large  variations  in  the  sulfur  content  would  indicate  that  a  sub- 
stance rich  in  sulfur  is  present  as  an  impurity. 

Decomposition  Products.  —  Among  the  products  of  decomposition 
are  amino-acids,  such  as  glycocoll,  leucin,  asparaginic  and  glutaminic 
acids,  alanin,  phenylalanin,  a-prolin,  etc.  Tyrosin  is  not  present. 

Reactions.  —  In  general  glue  is  not  precipitated  at  boiling  heat  by 
acids,  alum,  white  lead,  or  other  salts  of  the  metals.  It  is  precipitated 
by  tannic  acid  in  the  presence  of  salts  and  by  HgCk  in  the  presence  of 
NaCl  and  HC1.  On  boiling  glue  goes  over  into  a  modification  /3  glutin 
that  cannot  be  coagulated.  On  long  boiling  especially  in  the  presence 
of  acids,  glue  albumoses  and  glue  peptones  are  formed,  and  out  of  these 
by  further  splitting  amino-acids  are  obtained. 

Protective  Effect.  —  Gelatins  possess  an  extraordinary  protective 
effect,  and  are  in  this  regard  scarcely  surpassed  by  any  other  material. 
At  boiling  heat  the  gold  number  is  0.005  to  0.01.*  At  lower  temper- 
atures, i.e.,  80  degrees,  the  subdivision  is  not  so  great  and  the  solutions 
have  a  somewhat  smaller  gold  number.  Still  higher  gold  numbers  can  be 
obtained  if  gelatin  solutions  having  a  concentration  of  0.1  to  1  per 
cent  are  allowed  to  stand  over  night  and  are  diluted  to  T^  per  cent  on 
*  W.  Menz:  Zeit.  f.  phys.  Chemie,  66,  129-137  (1909). 


224  CHEMISTRY  OF  COLLOIDS 

the  following  day.  The  more  concentrated  the  solution  the  greater 
the  size  of  the  submicrons  and  therefore  the  smaller  the  protective 
effect. 

Osmotic  Pressure  of  Gelatin  Solutions.  —  The  determination  of 
the  osmotic  pressure  has  revealed  pronounced  hysteresis  phenomena  as 
shown  by  Lillie.*  Moore  and  Roaf  f  observed  a  greater  increase  in 
the  osmotic  pressure  with  a  rise  in  temperature  than  corresponds  to 
the  same  rise  in  solutions  of  crystalloids.  On  cooling,  the  original  pres- 
sure is  not  obtained  for  some  time,  but  remains  considerably  above  the 
normal.  Lillie  kept  a  gelatin  solution  in  the  ice  chest  for  two  days 
and  then  heated  a  part  of  the  solution  for  three  hours  at  65  to  70  de- 
grees. Both  solutions  were  brought  to  the  temperature  of  the  room  and 
the  pressure  measured.  The  solution  that  had  been  warmed  showed  a 
greater  pressure  than  the  other  but  after  a  time  both  columns  stood  at 
the  same  height.  The  equilibrium  was  reached  in  the  solution  that 
had  been  warmed  much  sooner  than  in  the  other.  The  observation  is 
in  good  agreement  with  the  general  behavior  of  the  two  solutions.  At 
first  the  one  that  had  been  warmed  contained  much  smaller  particles, 
thus  accounting  for  the  greater  pressure.  After  standing  some  time 
the  degree  of  dispersion  in  both  solutions  became  the  same. 

Graham  considered  glue  a  typical  colloid  because  it  was  so  easy  to 
obtain  both  the  solution  and  the  gel.  As  is  well  known  glue  distends  in 
water  to  form  a  gel,  and  on  warming  a  thick  liquid  is  obtained.  On 
cooling  the  solution  becomes  a  more  or  less  stiff  gel.  The  reaction  is 
reversible  as  long  as  the  heating  is  not  kept  up  for  too  great  a  length 
of  time,  in  which  case  chemical  reactions  set  in.  Changes  in  the  vis- 
cosity, elasticity,  and  in  the  addition  of  other  substances  have  been 
thoroughly  studied.  Of  special  interest  is  the  phenomenon  of  gelat- 
inization  itself. 

Ultramicroscopy  of  Gelatin  Solutions  and  Gels.  —  Pure  warm 
gelatin  solutions  appear  to  be  almost  homogeneous.  On  cooling  there 
is  formed,  according  to  the  concentration,  an  amicroscopical  or  a  sub- 
microscopical  heterogeneity.  The  maximum  size  of  the  particles  is 
reached  in  0.5  to  1  per  cent  solutions,  which  harden  to  porous  gels. 
The  farther  the  concentration  decreases  from  0.5  per  cent  the  smaller 
the  visible  particles  become,  and  the  more  mobile  the  liquid.  The 
greater  the  concentrations  over  0.5  per  cent  the  more  rigid  the  gels 
formed  on  cooling.  Submicrons  may  be  observed  in  solutions  from  0.1 
to  6  per  cent.  At  concentrations  greater  than  6  per  cent  or  less  than 
0.1  per  cent  only  amicrons  are  present. 

*  R.  S.  Lillie:  Amer.  Journ.  of  Physiol.,  20,  127-169  (1907). 
f  B.  Moore  and  H.  E.  Roaf:  Biochemical  Journ.,  2,  34  (1906). 


PROTEIN  BODIES  225 

If  warm  gelatin  solutions  are  considered  homogeneous,  cooling  causes 
the  appearance  of  a  new  phase,  and  the  process  is  similar  to  that  of 
crystallization  where  the  crystals  are  of  ultramicroscopical  dimensions, 
i.e.,  the  preparation  of  colloidal  gold,  silver,  salts,*  etc.  It  is  also 
similar  to  a  liquid  at  the  critical  point.  Weimarn  has  observed  that, 
similar  to  gelatin,  during  the  formation  of  crystalline  precipitates,  the 
size  of  the  particles  is  at  a  maximum  at  moderate  concentrations  and 
that  at  higher  concentrations  the  precipitate  is  a  gel.  These  observa- 
tions are  scarcely  comparable  to  the  hardening  of  gelatin  because  in 
the  latter  case  the  medium  is  always  water,  while  crystalline  precipi- 
tates are  formed  in  electrolytic  solutions,  where  the  concentration  is 
constantly  varying.  This,  of  course,  has  an  effect  on  the  size  of  the 
particles.  The  difference  between  gelatin  solutions  and  crystalliza- 
tion, or  solutions  at  the  critical  point,  is  that  in  the  first  case  microns, 
submicrons,  ultramicrons,  or  amicrons  are  formed  according  to  the 
concentration.  It  should  also  be  noted  that  during  the  cooling  of 
gelatin  solutions  visible  particles  made  up  of  smaller  amicrons  are 
present.  The  appearance  of  visible  particles  is  therefore  not  dependent 
upon  the  formation  of  a  gel  that  hangs  together.  The  closer  the  par- 
ticles are  together  the  greater  will  be  the  viscosity  of  the  solution. 
When  the  particles  are  close  enough  they  hold  together  and  form  a  gel, 
that  contains  a  great  deal  of  the  liquid  which  may  be  pressed  out. 

The  structure  of  gelatin  gels  has  been  studied  a  great  deal  micro- 
scopically. As  the  ordinary  gel  from  aqueous  solution  appears  to  be 
homogeneous  Butschli  and  also  Hardy  §  have  endeavored  to  make 
the  structure  visible.  The  latter  investigator  allowed  gelatin  hydrosols 
containing  alcohol  to  harden  and  viewed  the  process  under  the  micro- 
scope. In  dilute  solutions  drops  separated  out  on  cooling  that  united 
with  each  other  to  form  a  net-like  structure.  This  net  was  surrounded 
by  a  phase  that  contained  less  gelatin.  In  the  case  of  more  con- 
centrated solutions  an  enclosed  phase  richer  in  gelatin  was  formed 
which  contained  drops  of  the  water  alcohol  solution.  Butschli  J  made 
the  structure  visible  in  concentrated  gels  by  treating  them  with  alcohol 
or  chromic  acid,  etc. 

Pauli  §  has  shown  that  gelatin  is  materially  changed  by  such  treat- 
ment and  the  structure  must  therefore  be  considered  artificial.  It  has 
since  been  shown  that  gels  treated  with  these  reagents  have  different 

*  P.  P.  v.  Weimarn:  Koll.-Zeit.,  2-3. 

t  W.  B.  Hardy:  Zeit.  f.  phys.  Chemie,  33,  326-343  (1900). 

t  O.  Butschli:  Untersuchungen  iiber  mikroskopische  Schaume  und  das  Proto- 
plasma.  Leipzig  (1892). 

§  W.  Pauli:  Der  kolloidale  Zustand  und  die  Vorgange  in  der  lebendigen  Sub- 
stanz.  Braunschweig  (1902).  Naturwiss.  Rundschau,  17,  Nr.  25,  26,  27  (1902). 


226  CHEMISTRY  OF  COLLOIDS 

properties  from  the  gels  obtained  from  pure  water.  Hardy's  alcohol 
gels,  for  instance,  are  less  consistent  and  are  somewhat  milky.  At  the 
suggestion  of  the  author,  Bachmann  has  repeated  the  experiments  of 
Biitschli  and  Hardy,  and  found  that  there  is  a  marked  difference  be- 
tween the  structure  of  water  gels  and  those  treated  afterward  by  alcohol 
or  chromic  acid.  The  former  are  much  finer.  In  fact  gels  having  a 
concentration  of  over  6  per  cent  allow  no  differentiation  of  the  particles 
even  under  the  cardiod  ultramicroscope  with  sunlight.  That  there  is  a 
very  fine  discontinuity  is  revealed  by  the  fact  that  the  Tyndall  effect  is 
apparent.  The  refracted  light  suffers  linear  polarization. 

According  to  the  explanation  given  by  Bachmann  *  the  structure  of 
the  gels  obtained  by  Biitschli  is  due  to  a  conglomeration  of  particles 
forming  denser  units  that  are  separated  by  capillary  spaces  large  enough 
to  be  seen  under  an  ordinary  microscope.  A  distinct  structure  is 
visible  under  the  ultramicroscope  in  the  case  of  gels  from  solutions  of 
0.5  to  1  per  cent.  Flocks  of  microns  or  submicrons  may  be  distin- 
guished.! These  accumulations  have  different  shapes,  and  the  flocks 
must  be  regarded  as  groups  of  amicrons. 

The  formation  of  the  gel  itself  under  the  ultramicroscope  is  inter- 
esting and  has  been  studied  by  Menz,J  v.  Weimarn,§  and  Bachmann.* 
If  a  0.5  per  cent  solution  is  allowed  to  cool  a  multitude  of  submicrons 
can  be  detected,  which  grow  by  joining  together  to  form  flocks.  The 
particles  are  not  at  rest  but  have  a  vibratory  motion  not  quite  so  great 
as  they  have  in  the  hydrosol.  Gradually  the  solidity  increases  and  the 
motion  becomes  less  and  less.  The  rate  of  solidification  may  be  regu- 
lated by  judicious  cooling.  The  process  has  a  certain  similarity  to 
the  separation  at  the  critical  points  of  liquids.  At  the  suggestion  of 
the  author  v.  Lepkowski  1f  investigated  the  critical  phenomena  under  the 
ultramicroscope.  During  the  cooling  an  intense  lighting  up  of  the 
field  is  observed  shortly  before  the  separation,  and  multitudes  of  not 
clearly  defined  submicrons  are  present.  Suddenly  a  new  phase  appears 
in  the  form  of  tiny  drops.  On  warming  a  motion  on  the  surface  of  these 
drops  can  be  seen  and  they  either  become  invisibly  small,  or  the 
contours  gradually  fade  and  the  place  where  the  drops  were  is  now 
characterized  for  some  little  time  by  a  glimmering  zone.  Evidence  of 

*  W.  Bachmann:  Inaug.-Diss.  Gottingen  (1911);  Zeit.  f.  anorg.  Chemie,  73,  125 
172  (1911). 

t  P.  v.  Weimarn:  Koll.-Zeit.,  10,  132  (1912). 

t  Menz:  I.  c. 

§  P.  v.  Weimarn:  Koll.-Zeit.,  4,  133  (1909);  6,  277  (1910).  Grundziige  der  Dis- 
persoidchemie.  Dresden  (1911). 

If  W.  v.  Lepkowski:   Zeit.  f.  pnys.  Chemie,  75,  608-614  (1911). 


PROTEIN  BODIES  227 

the  small  diffusion  in  the  liquid  is  afforded  by  the  fact  that  on  cooling 
the  drops  may  be  obtained  on  the  spot  where  they  disappeared.  In 
fact  two  particles  that  were  prevented  from  uniting  by  the  warming 
may  be  so  far  restored  that  they  will  unite  after  the  cooling  process  has 
been  carried  out.  In  contradistinction  to  the  case  of  gelatin  submicrons 
from  critical  systems  unite  to  form  a  homogeneous  phase  even  after 
the  cooling.  The  drops  are  circular,  large,  and  have  no  such  variations 
in  form  as  are  so  prominent  in  the  case  of  gelatin  particles. 

In  regard  to  the  above  observations  on  gelatin  under  the  ultrami- 
croscope  it  must  be  borne  in  mind  that  the  finest  structure  seen  is  npt 
by  any  means  the  finest  present  in  the  system.  The  resolving  power  of 
the  ultramicroscope  is  such  that  if  two  particles  are  closer  together 
than  one-half  the  wave  length,  the  two  would  appear  to  be  one  particle. 
How  conclusions  may  be  made  with  regard  to  finer  structures  than 
this,  has  been  discussed  under  the  heading,  Gels  of  Silicic  Acid.* 

Gelatinization  and  Distention  f  (Swelling) 

Gelatinization  and  distention  have  been  the  subjects  of  many  physi- 
cal investigations.  Changes  in  viscosity,  elasticity,  the  heat  of  reac- 
tion during  distention,  and  the  influence  of  additions  on  the  temperature 
of  gelatinization  have  also  been  carefully  studied. 

Distention.  —  In  water  gelatin  distends  to  form  a  gel,  and  takes  up 
8  to  10  times  its  weight  of  water.  Most  of  this  water  is  given  up  again 
in  an  atmosphere  saturated  with  water  vapor,  and  equilibrium  is  again 
established  when  the  water  is  about  50  per  cent  of  the  weight  of  the  dry 
gelatin.  This  water  is  much  more  firmly  bound  than  that  actually 
necessary  for  distention,  and  is  given  up  but  slowly  under  reduced 
aqueous  vapor  pressure.  This  is  determined  by  the  heat  of  reaction 
during  distention,  and  by  the  rate  of  distention.  According  to  Wiede- 
mann  and  Liideking  |  the  heat  of  distention  is  5.7  cals.  per  gram  of 
dried  gelatin.  On  the  contrary  during  the  liquefaction  of  gelatin  heat  is 
taken  up.  The  process  is  comparable  to  the  dissolution  of  many  salts 
whereby  heat  is  at  first  given  out  owing  to  the  formation  of  hydrates, 
but  afterwards  heat  is  taken  up  as  the  dilution  is  continued.  The  heat 
of  distention  depends  upon  the  degree  of  moisture  in  the  hydrogel. 
This  is  shown  by  table  39  taken  from  Rodewald's§  investigations. 
After  the  amount  of  water  has  risen  to  about  20  per  cent  not  much 

*  P.  Bohi:  Inaug.-Diss.,  27.     Zurich  (1911). 
t  Journ.  Am.  Chem.  Soc.,  37,  1295. 

$  E.  Wiedemann  und  Ch.  Liideking:  Wiedemanns  Annalen  d.  Phys.  u.  Chem. 
N.  F.,  25,  145-153  (1885). 

§  H.  Rodewald:  Zeit.  f.  phys.  Chemie,  24,  206  (1897). 


228 


CHEMISTRY  OF  COLLOIDS 


more  heat  is  given  out.  Hand  in  hand  with  a  large  heat  of  distention 
goes  a  large  contraction,  particularly  in  the  first  stages.  The  researches 
of  Hofmeister,*  and  of  Pascheles  f  on  the  rate  of  distention  indicate 
that  the  water  at  first  added  is  firmly  bound.  The  water  is  taken  up 
rapidly  at  first  and  then  more  slowly  toward  the  end. 

TABLE  39 


%  H20. 

Q. 

0.23 

28.11 

3.23 

20.97 

8.16 

12.43 

12.97 

7.37 

19.52 

2.91 

Solidity.  —  Dried  glue  is  characterized  by  an  extraordinary  firm- 
ness. This  is  exemplified  by  the  strength  glue  exhibits  in  holding  two 
pieces  of  wood  or  glass  together.  In  the  latter  case  the  glass  is  often 
torn  at  the  surface  before  the  two  pieces  can  be  forced  apart.  Reliefs 
in  glass  surfaces  are  made  in  the  industries  by  this  method.  The  ex- 
periment shows  the  enormous  adhesion  between  glue  and  other  amor- 
phous substances.  The  firmness  decreases  with  increasing  water 
content. 

On  freezing  moist  gelatin  suffers  a  change  of  state,  and  after  the 
thawing  the  properties  are  somewhat  different.  This  is  apparent  in 
the  structure  under  the  microscope,  in  the  decrease  of  adhesive  power, 
and  also  in  decreased  distention;  see  page  64. 

That  a  considerable  portion  of  the  water  in  gelatin  gels  is  weakly 
bound  is  shown  by  the  researches  of  Butschli.t  The  gel  was  rubbed 
into  a  paste,  put  into  an  unglazed  porcelain  cell  and  the  latter  subjected 
to  reduced  pressure  by  a  water  pump.  In  this  manner  he  was  able  to 
remove  all  but  25  per  cent  of  the  water  when  gels  of  5  to  10  per  cent 
concentration  were  employed.  Even  when  the  warm  solution  was 
poured  into  the  cell  and  allowed  to  solidify  the  water  could  be  withdrawn 
by  the  same  process.  It  could  not  be  carried  out,  however,  when  the 
concentration  was  20  per  cent.  This  agrees  very  well  with  the  prop- 
erty that  gels  from  dilute  solutions  often  have  of  contracting  and  giving 
up  a  part  of  the  water  contained,  and  also  with  the  observation  that 
distended  gelatin  will  give  up  a  portion  of  its  water  even  in  an  atmos- 
phere saturated  with  water  vapor. 

*  F.  Hofmeister:  Archiv.  f.  experim.  Pathol.  u.  Pharmakol.,  27,  395-413  (1890). 
t  W.  Pascheles  (Pauli):  Pfliigers  Archiv.  f.  d.  ges.  Physiol.,  67,  219-239  (1897). 
j  0.  Biitschli:  Tiber  den  Bau  queUbarer  Korper.  Gottingen,  22-26  (1896). 


PROTEIN  BODIES  229 

The  Displacement  of  Water  by  Other  Liquids.  —  Biitschli  found 
that  it  was  very  easy  to  replace  water  in  a  gelatin  gel  with  alcohol,  and 
this  again  by  chloroform,  turpentine  or  xylol  (xylene).  In  this  man- 
ner gels  are  obtained  that  are  turbid,  solid,  and  do  not  shrink  a  great 
deal  on  drying.  They  are  porous,  chalk  white  and  opaque.  This  is 
similar  to  an  experiment  carried  out  by  Graham  on  silicic  acid  gels, 
and  shows  a  close  relation  between  gelatin  gels  and  those  purely  inor- 
ganic, although  the  latter  may  have  a  somewhat  finer  structure.  The 
investigation  of  the  vapor  tension  curves  during  the  drying  would 
doubtless  give  valuable  information  about  the  size  of  the  enclosed 
spaces. 

The  Distention  of  Gelatin  and  Other  Colloids  in  the  Presence  of 
Electrolytes.  —  Through  the  investigations  of  Hofmeister,*  Pauli,f 
Spiro,  t  W.  Ostwald,§  and  Fischer,  the  distention  of  gelatin  in  the 
presence  of  electroytes  has  become  well  known.  The  last-named 
author  has  shown  that  not  only  gelatin  but  also  fibrin,  muscle  and 
the  substance  in  the  eyes  of  cattle  behave  similarly.  He  has  also  dis- 
covered a  relation  between  the  distention  in  colloids  and  the  abnormal 
accumulation  of  water  in  animal  fibers. 

From  the  work  of  Spiro  we  learn  that  acid  and  alkalis  have  a  much 
more  pronounced  distending  effect  than  water.  Wo.  Ostwald  has  shown 
that  acids  vary  in  their  effectiveness.  The  following  series  is  arranged 
in  descending  order  of  the  effectiveness. 

Hydrochloric  acid  >  nitric  acid  >  acetic  acid  >  sulfuric  acid 

>  boracic  acid. 

Fischer  has  shown  that  the  acid  and  alkali  distention  of  gelatin,  fibrin, 
muscles  and  the  substance  in  the  eyes  of  animals  is  depressed  by  the 
presence  of  salts.  Chlorides,  bromides  and  nitrates  have  a  much  less 
depressing  effect  than  acetates,  sulfates  or  citrates.  This  series  agrees 
in  the  main  but  not  completely  with  that  obtained  in  the  precipitation 
of  proteids.  According  to  Fischer  If  the  abnormal  accumulation  of  water 
in  the  tissues  is  not  caused  by  differences  in  blood  pressure,  but  by 
acids  formed  in  the  tissues,  either  because  of  disturbances  in  the  cir- 
culation, or  by  infections. 

*  F.  Hofmeister:  Archiv.  f.  experim.  Pathol.  u.  Pharmakol.,  27,  395-413  (1890); 
28,  210-238  (1891). 

t  W.  Pauli:  Pfliigers  Archiv.  f.  d.  ges.  Physiol.,  67,  219-239  (1897);  71,  333- 
356  (1898). 

J  K.  Spiro:  Hofmeisters  Beitrage  z.  chem.  Physiol.  u.  Pathol.,  6,  276-296  (1904). 

§  Wo.  Ostwald:  Pfliigers  Archiv.  f.  d.  ges.  Physiol.,  108,  563-589  (1905);  111, 
581-606  (1906). 

T  M.  H.  Fischer:  (Edema.    New  York  (1910). 


230  CHEMISTRY  OF  COLLOIDS 

If  gelatin  is  pricked  with  a  needle  that  has  been  previously  dipped  in 
formic  acid  and  the  entire  mass  covered  with  water,  those  places  that 
have  been  pricked  will  distend  much  faster  than  the  others.  The 
swollen  places  resemble  swellings  on  the  human  skin  caused  by  the 
sting  of  insects.  Just  as  sulfates,  acetates  and  especially  citrates  may 
be  used  to  reduce  the  distention  of  gelatin  so  may  they  be  employed  to 
reduce  the  accumulations  of  water  in  the  tissues. 

Diffusion  Through  Gelatin,  Ultrafiltration 

Graham  *  and  other  investigators  have  found  that  gels  offer  very 
little  resistance  to  the  passage  of  electrolytes.  Nell,f  Bechhold  and 
Ziegler  J  have  recently  shown  that  only  dilute  gels  offer  a  negligible  re- 
sistance, while  concentrated,  on  the  contrary,  show  considerable.  This 
is  quite  in  accordance  with  the  knowledge  obtained  through  the  ultra- 
microscope 'that  the  structure  of  gels  is  granular.  Little  resistance  is 
offered  by  the  large  spaces  in  a  thin  gel  to  the  passage  of  electrolytes. 
A  more  concentrated  gel  would  entail  the  passage  of  the  molecules 
through  the  walls  made  of  gelatin,  and  thus  cause  a  greater  resistance. 

Colloids  in  general  do  not  diffuse  through  gels.  The  retarded  diffu- 
sion is  probably  connected  with  adsorption.  Thick  collodion  mem- 
branes generally  effectually  prevent  the  passage  of  ultramicrons. 
Diffusion  is  usually  very  complicated  in  gels  where  the  size  of  the  pores, 
ultramicrons,  adsorption,  charge  and  discharge  of  the  particles  all 
play  a  part.  Liesegang§  has  devised  a  very  pretty  experiment  in  the 
diffusion  of  silver  nitrate  through  gels  containing  chromium.  A  system 
of  rings  is  formed  around  the  drops  of  silver  nitrate,  where  the  distance 
of  the  rings  from  the  drops  increases  with  the  progress  of  the  diffusion. 
OstwaldH  has  offered  an  explanation  for  this,  but  Bechhold,||  and 
Liesegang  **  contend  that  the  process  is  more  complicated.  Bechhold  ft 
and  Ziegler  allowed  salts  that  would  give  a  precipitate  to  diffuse  against 
each  other,  and  observed  that  the  precipitate  often  completely  obstructed 
the  diffusion.  By  melting  the  jelly  the  process  progressed  further. 

Ultrafiltration  by  Gelatin.  —  Bechhold  f|  observed  that  hardened 
gelatin  allowed  solutions  of  crystalloids  to  pass  through  unobstructed, 

*  Th.  Graham:  Liebigs  Annalen,  121,  5,  29  (1862). 

t  P.  Nell:  Annalen  d.  Phys.  (4),  18,  323-347  (1905). 

t  H.  Bechhold  und  J.  Ziegler:   Zeit.  f.  phys.  Chemie,  56,  105-121  (1906). 

§  R.  Liesegang:  Liesegangs  photogr.  Archiv.,  321-326  (1896).  Chemische  Reak- 
tionen  in  Gallerten.  Dusseldorf  (1898). 

If  W.  Ostwald:  Lehr.  d.  allg.  Chemie  (2  Aufl.),  2,  11,  778  ff. 

II  H.  Bechhold:  Zeit.  f.  phys.  Chemie,  52,  185-199  (1905). 
**  R.  Liesegang:  Zeit.  f.  phys.  Chemie,  69,  444-447  (1907). 
ft  H.  Bechhold  und  J.  Zeigler:  Annalen  d.  Phys.  (4),  20,  900-918  (1906). 
8  H.  Bechhold:  Zeit.  f.  phys.  Chemie,  60,  257-318  (1907). 


PROTEIN  BODIES  231 

but  held  back  colloidal  particles,  just  as  collodion  filters  do.  These 
facts  indicate  a  granular  or  at  least  an  open  net  structure  of  the  gelatin, 
the  walls  of  which  offer  a  great  resistance  to  the  filtration.  Beside  the 
considerations  discussed  under  diffusion,  other  factors  must  act.  Ac- 
cording to  Bechhold  *  the  pores  of  his  filter  were  much  larger  than  the 
ultramicrons  that  they  held  back.  It  is  not  impossible  that  the  im- 
permeability of  the  filter  for  colloids  is  connected  with  a  dynamic  proc- 
ess on  the  surface,  and  that  the  kinetic  theory  may  throw  some  light 
on  the  phenomenon. 

B.  Hemoglobin 

Hemoglobin  and  Oxyhemoglobin 

The  red  color  of  the  blood  is  occasioned  partly  by  the  presence  of 
hemoglobin,  a  protein  body,  and  partly  by  oxyhemoglobin,  a  com- 
pound of  oxygen  and  hemoglobin.  In  the  red  corpuscles  the  hemo- 
globin is  surrounded  by  another  substance,  and  perhaps  also  by  a 
membrane.  The  material  of  this  substance  is  composed  largely  of  the 
so-called  lipoids,  chloresterin,  lecithin,  etc.  The  liquid  in  the  red 
corpuscles  is  isotonic  with  a  sodium  chloride  solution  having  a  con- 
centration of  9  parts  per  million.  In  this  latter  solution  the  corpuscles 
remain  unchanged,  while  in  one  more  concentrated  they  shrink,  or  swell 
in  one  more  dilute.  This  distention  may  proceed  so  far  that  the  hemo- 
globin separates  out  from  the  other  liquid  in  the  corpuscles  and  goes 
into  the  outer  solution.  This  process  is  known  as  hemolysis.  Freezing 
and  the  influence  of  some  reagents,  such  as  ether,  chloroform,  saponin, 
may  also  cause  hemolysis. 

Hemoglobin  dissolved  in  water  will  not  diffuse  through  membranes, 
and  the  solution,  according  to  the  definition  of  Graham,  is  colloidal.f 
It  is  also  worthy  of  note  that  Bechhold's  filter  of  suitable  thickness  will 
prevent  the  diffusion  of  the  hemoglobin.  In  fact  Bechhold  employs 
this  means  of  determining  the  permeability  of  his  membranes. 

Hemoglobin  crystals  from  the  blood  of  different  animals  do  not 
have  the  same  composition  nor  properties.  Bohr  assumes  that  there 
are  different  sorts  of  hemoglobin  in  the  blood  of  a  single  species,  but 
according  to  Hiifner  there  is  only  one  hemoglobin  in  the  blood  of 
cattle.  Noteworthy  is  the  iron  content,  which  is  connected  with  the 
taking  up  of  oxygen  by  the  hemoglobin.  Indeed  this  property  of 
hemoglobin  to  take  up  oxygen,  carbon  monoxide,  and  other  gases  is  of 
highest  importance  to  form  oxyhemoglobin,  or  carbon  monoxide  — 
hemoglobin.  Hufner  t  has  found  that  1  mol.  of  hemoglobin  will  take 

*  H.  Bechhold:  Zeit.  f.  phys.  Chemie,  64,  328-342  (1908). 

t  J.  Lemanissier:  Etudes  des  corps  ultramiscroscopiques.     Paris  (1906). 

t  C.  Bohr:  Centralbl.  f.  Physiol,  4,  249-252  (1890). 


232  CHEMISTRY  OF  COLLOIDS 

up  1  mol.  oxygen  at  higher  pressures;  or  1  mol.  oxygen  is  taken  up  for 
every  mol.  iron  in  the  hemoglobin. 

Oxyhemoglobin  crystallizes  much  more  easily  than  hemoglobin  and 
may  be  prepared  in  the  following  way. 

Well-washed  blood  from  dogs  or  horses  is  added  to  2  volumes  of  water 
and  shaken  with  ether.  The  excess  of  ether  is  poured  off  and  the 
amount  dissolved  allowed  to  evaporate  in  a  shallow  dish  at  0°  C.  The 
liquid  is  allowed  to  stand  for  two  to  three  days  after  about  one-fourth 
volume  of  alcohol  has  been  added.  The  product  is  purified  by  re- 
crystallization  from  aqueous  solution  by  the  addition  of  alcohol.  The 
crystals  have  about  the  following  composition. 

Per  cent 

C 53. 8  to  54. 7 

H 6. 9  to    7.3 

N 16     to  17. 5 

S 0.4to    0.6 

0 19     to  22 

Fe 0.33 

The  absorption  spectrum  of  oxyhemoglobin  is  markedly  different 
from  that  of  hemoglobin.  The  former  has  two  sharp  well-defined 
absorption  bands  between  D  and  E,  while  the  latter  has  only  one. 
Such  differences  in  the  absorption  spectra  cannot  be  explained  on  the 
grounds  of  a  difference  in  state,  nor  in  the  degree  of  dispersion  of  the 
hemoglobin.  Rather  it  indicates  a  change  in  the  chemical  composition 
of  the  two  substances. 

Oxyhemoglobin  will  give  up  its  oxygen  very  easily;  is  easily  re- 
duced in  other  words,  and  this  fact  is  of  great  importance  in  the  oxi- 
dation processes  in  the  body.  Oxygen  is  taken  up  in  the  lungs  and 
transported  to  the  tissues  where  the  oxidation  processes  go  on.  Num- 
bers of  measurements  have  been  made  on  the  amount  of  oxygen  taken 
up  by  the  blood  without  any  very  great  agreement  among  the  different 
experimenters.  It  would  seem  that  carefully  prepared  solutions  of 
hemoglobin  might  lead  to  more  satisfactory  results;  because  in  the 
blood,  which  is  a  mixture  of  several  protein  bodies,  adsorption  of  the 
oxygen  in  the  disperse  phase  must  play  a  part  as  well  as  chemical  com- 
bination. Ostwald  *  has  shown  that  the  adsorption  isotherms  are 
suitable  for  describing  the  quantitative  adsorption  of  the  oxygen. 

Methemoglobin.  —  On  long  standing  and  also  under  the  influence 
of  a  series  of  different  reagents  oxyhemoglobin  goes  over  into  another 
modification,  known  as  methemoglobin.  Oxidizing  and  reducing 
agents,  as  well  as  many  indifferent  substances,  effect  or  accelerate  the 
change.  The  transition  was  previously  regarded  as  a  reduction,  but 
*  Wo.  Ostwald:  Koll.-Zeit.,  2,  264-272,  294-301  (1908). 


PROTEIN  BODIES  233 

Hufner  *  has  demonstrated  it  to  be  an  oxidation  process.  The  product 
is  much  more  stabl  than  oxyhemoglobin,  does  not  give  up  its  oxygen 
in  vacuum,  and  may  be  crystallized  like  oxyhemoglobin.  Methemo- 
globin  has  a  different  absorption  spectrum  in  acid  and  alkaline  solution. 
Both  of  these  are  different  from  that  of  oxyhemoglobin.  According 
to  Kiihne  t  and  Preyer  J  oxyhemoglobin  and  methemoglobin  are  acids, 
while  hemoglobin  is  not.  Oxyhemoglobin  is  much  more  insoluble  than 
hemoglobin.  These  differences  indicate  quite  clearly  that  oxyhemo- 
globin is  chemically  different  from  hemoglobin,  and  that  the  former  is 
not  merely  an  adsorption  compound  of  oxygen  with  the  latter. 

Carbon  Monoxide-Hemoglobin.  —  As  in  the  case  of  oxygen  hemo- 
globin forms  a  compound  with  carbon  monoxide,  whereby  the  color 
becomes  cherry  red.  The  crystals  of  this  substance  show  a  weak  but 
beautiful  pleochroism,  purple,  red  and  white.  The  absorption  bands 
are  similar  to  those  of  oxyhemoglobin,  but  are  displaced  somewhat 
more  toward  E.  Carbon  monoxide  is  given  up  with  difficulty,  and  this 
accounts  for  the  fact  that  carbon  monoxide  can  displace  oxygen  in 
the  blood  even  when  the  latter  gas  is  present  in  moderate  concentra- 
tion, hence  the  poisonous  effects  of  carbon  monoxide  in  the  blood. 
Twenty-seven  per  cent  of  the  hemoglobin  will  be  united  to  carbon 
monoxide  when  the  latter  is  present  in  the  air  at  a  concentration  of  only 
0.05  per  cent,  and  the  partial  pressure  of  the  oxygen  is  545  times  greater 
than  that  of  the  carbon  monoxide.  Carbon  monoxide-hemoglobin  is 
much  more  difficult  to  turn  into  methemoglobin.  It  is  also  much  more 
stable  toward  reagents  than  oxyhemoglobin.  Many  substances  that 
change  oxyhemoglobin  into  methemoglobin  do  not  affect  carbon  mon- 
oxide-hemoglobin . 

Molecular  Weight  of  Hemoglobin 

Hufner  §  has  shown  that  1  gram  hemoglobin  unites  with  1.338  cc. 
carbon  monoxide  ( =  0.00167  g.)  at  normal  conditions.  On  the  assump- 
tion that  the  combination  takes  place  in  molecular  amounts  of  each, 
the  molecular  weight  of  hemoglobin  would  be  16,721.  The  iron 
content  of  the  blood  is  0.336  per  cent.  On  the  assumption  that  one 
molecule  of  hemoglobin  contains  one  atom  of  iron  the  molecular 
weight  would  be  16,666,  practically  the  same  value  as  that  obtained 

*  G.  Hufner  und  J.  Otto:  Zeit.  f.  physiol.  Chemie,  7,  65-70  (1882).  Ders.  und 
R.  Kiilz:  Ibid.,  7,  366-374  (1883). 

t  W.  Kiihne:  Virchows  Archiv.,  34,  423-436  (1865). 

t  W.  Preyer:  Centralbl.  f.  d.  med.  Wiss.,  273-275  (1867);  Pfliigers  Archiv.  f.  d. 
ges.  Physiol.,  1,  395-454  (1868). 

§  G.  Hufner:  Englemanns  Archiv.  f.  Physiol.,  Physiol.  Abt.,  130-176  (1894); 
217-224  (1903). 


234 


CHEMISTRY  OF  COLLOIDS 


on  the  basis  of  the  taking  up  of  carbon  monoxide.  It  is  quite  possible 
that  one  molecule  of  hemoglobin  contains  more  iron  than  we  have 
assumed,  i.e.,  n  molecules  of  iron.  Hemoglobin  would  therefore  take 
up  n  mols  carbon  monoxide  to  one  hemoglobin,  and  have  a  molecular 
weight  n  times  the  above  number.  In  order  to  decide  this  question 
Hiifner  and  Gansser  *  measured  the  osmotic  pressure  of  a  hemoglobin 
solution  with  the  help  of  a  parchment  bag  employed  by  Schleicher 
and  Schull.  Fig.  33  represents  the  apparatus. 


FIG.  33.    Hiifner's  apparatus  for  the  determination  of  osmotic  pressure. 

A  parchment  bag,  100  mm.  long  and  16  mm.  wide  is  connected  with 
the  manometer  by  means  of  the  capillary  tube,  c.  A  funnel  tube,  I,  is 
held  in  place  by  the  arm  of  the  ring  stand,  p.  The  capillary  under  the 
two-way  stop  cock,  z,  has  a  small  bulb  of  about  1  cc.  capacity.  The 
bag  is  softened  with  water,  securely  bound  on  the  glass  tube,  g,  and 
covered  with  a  thick  layer  of  picene  by  melting  it  over  a  burner. 
The  manometer  is  filled  by  suction  at  z.  The  bag  is  filled  with  hemo- 
globin through  the  funnel  and  the  liquid  is  allowed  to  flow  out  at  z. 

*  G.  Hiifner  und  E.  Gansser:  Engelmanns  Archiv.  f.  Physiol.,  Physiol.  Abt., 
209-216  (1907). 


PROTEIN  BODIES  235 

z  is  now  opened  to  the  manometer  and  h  is  closed.  The  beaker  is 
filled  with  water  and  the  apparatus  allowed  to  stand.  In  18  to  24 
hours  the  mercury  will  have  risen  to  its  maximum.  The  volume,  how- 

ever, has  increased  by  osmosis  and  it  is  necessary  to  multiply  by  -  in 

order  to  get  the  proper  pressure.  The  molecular  weight  may  be  cal- 
culated by  the  formula 

,,  _  22.41  (1  +  0.00366  1)  0.760  c 

~v~ 

where  c  is  the  amount  of  substance  dissolved  in  a  liter,  and  pr  is  the 
corrected  pressure  in  millimeters  of  mercury.  For  further  details  refer- 
ence must  be  made  to  the  original  article. 

The  average  of  four  determinations  in  the  case  of  the  blood  of  horses 
gave  a  molecular  weight  of  15,115,  while  the  average  of  ten  determina- 
tions with  the  blood  of  cattle  gave  16,321.  In  the  last  case  the  mini- 
mum and  maximum  were  15,500  and  18,370  respectively. 

Size  of  the  Hemoglobin  Molecule 

The  agreement  of  the  molecular  weight  found  with  that  calculated 
from  chemical  data  is  remarkable.  We  have  here  an  excellent  example 
for  showing  how  the  same  point  may  be  arrived  at  by  two  entirely 
different  methods. 

Because  of  its  behavior  toward  dialysis  and  ultrafiltration  hemo- 
globin in  solution  must  be  considered  a  colloid.  On  the  other  hand, 
the  ease  with  which  oxyhemoglobin  can  be  crystallized  would  place 
these  substances  with  the  crystolloids.  All  methods  give  a  molecular 
weight  of  over  16,000  for  hemoglobin  from  the  blood  of  cattle.  But 
these  enormous  molecules  will  not  diffuse  through  membranes,  and 
Bechhold  has  shown  that  they  can  be  filtered  off  by  means  of  collodion 
filters  made  in  glacial  acetic  acid.  We  are,  therefore,  led  to  the  con- 
clusion that  the  molecule  of  oxyhemoglobin  and  the  ultramicron  is 
one  and  the  same  particle.  Similar  cases  are  presented  in  the  dye- 
stuff  domain,  but  because  of  electrolytic  dissociation  the  determined 
and  the  calculated  molecular  weight  do  not  agree  so  well  as  in  the  case 
under  discussion. 

Reinganum  *  has  submitted  the  following  formula  for  the  determi- 
nation of  the  diameter  of  the  molecules. 


*  See  R.  Lorenz:  Zeit.  f.  phys.  Chemie,  73,  253  (1910). 


236  CHEMISTRY  OF  COLLOIDS 

Here  M  is  the  molecular  weight,  S  the  specific  gravity  at  boiling  tem- 
perature, -o-  is  the  molecular  volume  that  can  be  approximately  cal- 

culated from  Kopp's  rule.     Under  the  assumption  that  the  formula 
given  for  the  hemoglobin  in  the  blood  of  the  dog  by  Jaquet  is  correct, 


the  diameter  of  the  hemoglobin  molecule  would  be 

2.3  -  2.5  MM, 

according  to  whether  we  take  Kopp's  lowest  or  highest  value  for  the 
atomic  volumes  of  0  and  N. 

It  is  worthy  of  note  that  the  determinations  of  the  diameter  of  ami- 
crons  in  gold  solutions  have  given  values  similar  to  the  above.*  In 
other  words  there  is  no  great  difference  between  the  room  taken  up  by 
the  gold  amicrons  and  the  molecules  of  hemoglobin.  Gold  particles 
may  be  either  larger  or  smaller  than  the  molecules  of  hemoglobin. 

C.  CASEIN 

Casein  belongs  to  the  true  protein  bodies,  and  specifically  to  the 
group  of  phosphorglobulins.  By  pepsin  digestion  these  give  pseudo- 
nuclein  a  substance  that  contains  phosphorus.  Pseudonuclein  becomes 
dissolved  by  further  digestion  and  differs  from  the  nuclein  of  the 
nucleoproteids  in  its  decomposition  products.  The  phosphorglobins 
were  previously  classed  with  the  nucleoproteids,  but  the  phosphorus 
content  is  about  the  only  thing  these  two  groups  have  in  common. 
The  phosphorglobulins  have  also  nothing  to  do  with  the  cell  nucleus 
out  of  which  the  nucleoproteids  are  obtained. 

Casein  is  the  most  important  protein  body  in  milk.  Besides  its 
chief  function  as  nitrogenous  food  it  plays  the  important  role  of  pro- 
tective colloid  for  the  fat  particles  and  also  for  calcium  phosphate  with 
which  it  forms  a  colloidal  combination.  It  keeps  the  fat  particles  in  a 
fine  state  of  emulsion  and  prevents  them  from  uniting  into  clots,  prob- 
ably due  to  the  formation  of  a  fine  membrane  around  the  particle. 
Casein  may  be  precipitated  from  milk  by  acids.  It  is  insoluble  in 
water  and  also  in  the  solutions  of  neutral  salts,  except  sodium  fluoride 
and  potassium  oxalate.  Casein  has  acid  properties,  drives  carbon 
dioxide  out  of  carbonates,  and  is  soluble  in  alkalis.  The  ammoniacal 
solution  possesses  a  high  protective  effect  toward  colloidal  gold.  The 
gold  number  0.01  indicates  that  it  belongs  therefore  to  the  protective 
colloids  of  the  first  class.  The  behavior  toward  bases  has  been  care- 

*  R.  Zsigmondy:  Zeit.  f.  phys.  Chem.,  66,  65-76  (1906). 


PROTEIN   BODIES  237 

fully  investigated  by  Laqueur  and  Sackur,*  and  also  by  Robertson.f 
The  two  former  concluded  from  their  work  that  the  proteid  ion  is  re- 
sponsible for  the  increased  viscosity  of  proteid  solutions. 

Pure  casein  solutions  do  not  curdle  on  boiling.  Acids  do  not  cause 
complete  precipitation  unless  the  temperature  is  raised  to  the  boiling 
point.  A  slight  excess  of  calcium  hydroxide  also  causes  turbidity  and 
coagulation  on  warming.  The  turbid  accumulation  dissolves  again 
when  the  temperature  is  lowered.  Concentrated  solutions  of  calcium 
and  casein  become  covered  with  a  film  on  boiling  just  as  milk  does. 

Acid  and  Rennet  Coagulation.  —  Acetic  and  mineral  acids  precipi- 
tate casein.  The  precipitate  dissolves  again  in  an  excess  of  the  acid, 
and  this  behavior  is  in  accordance  with  the  amphoteric  character  of 
casein  as  a  member  of  the  proteid  group  of  substances.  Rennet,  an 
enzyme  from  the  stomach  of  a  calf,  causes  another  sort  of  coagulation 
in  casein  solutions  containing  calcium.  This  is  of  industrial  importance 
in  the  making  of  cheese.  Calcium-free  casein  solutions  are  not  pre- 
cipitated by  rennet.  That  it  causes  a  change  to  take  place,  however, 
is  manifested  by  the  fact  that  calcium  ion  added  afterward  causes  im- 
mediate precipitation. 

Occurrence  in  Milk.  —  Casein  calcium,  or  a  colloidal  combination 
of  casein  and  calcium  phosphate,  is  present  in  milk  in  the  form  of  sub- 
microns.  They  are  there  in  enormous  numbers;  cow's  milk  contains 
3  to  6  billions  per  cubic  centimeter.  Under  the  assumption  of  cubical 
form  and  a  complete  filling  of  the  space  the  linear  dimensions  would 
be  130  to  170  JJL/JL.  The  particles  are  therefore  fairly  large  and  can  be 
seen  with  artificial  light.  The  size  of  the  particles  agrees  with  the  ex- 
perimental fact  that  they  may  be  retained  by  unglazed  porcelain  filters, 
and  casein  can  be  separated  from  milk  by  this  means.  Wiegner  J  has 
shown  that  the  number  of  submicrons  in  cow's  milk  is  remarkably  con- 
stant. Human  milk  is  easily  distinguished  from  that  of  the  cow  be- 
cause the  number  of  submicrons  visible  under  the  ultramicroscope  is 
much  greater  in  the  latter. 

If  it  is  necessary  to  prevent  the  precipitation  or  coagulation  of  casein 
by  acids  a  protective  colloid  stable  against  acids  must  be  added,  such 
as  gelatin,  or  albumin.  According  to  J.  Alexander  §  these  additions 
are  necessary  in  the  preparation  of  ice  cream.  If  the  protective  colloid 

*  E.  Laqueur  und  O.  Sackur:  Hofmeisters  Beitrage  z.  chem.  Physiol.  u.  Pathol.  3, 
193-224  (1903). 

t  T.  B.  Robertson:  Journ.  of  Physic.  Chemistry,  11,  542-552  (1907);  12,  473- 
483  (1908). 

t  G.  Wiegner:  Koll.-Zeit.,  8,  227-232  (1911). 

§  Jerome  Alexander:  Journ.  of  Soc.  of  Chem.  Industry,  28,  280  (1909);  Journ. 
of  Amer.  Med.  Assoc.,  55,  1196-1198  (1910). 


238 


CHEMISTRY  OF  COLLOIDS 


is  not  added  the  ice  cream  becomes  somewhat  granular  and  not  quite 
so  pleasant  to  the  taste. 

Another  interesting  point  raised  by  this  same  author  is  that  of  the 
comparison  of  human  and  cow's  milk  for  children.  The  cow's  milk  is 
usually  diluted  with  water  and  the  necessary  amount  of  milk  sugar 
added.  This  is,  however,  not  sufficient  to  render  the  milk  similar  in 
content  to  that  of  the  human  species.  A  striking  difference  between 
the  two  sorts  is  illustrated  by  the  fact  that  human  milk  is  not  so  easily 
coagulated  by  acids  as  cow's  milk.  The  latter  contains  more  casein 
and  less  albumin  as  will  be  seen  from  the  following  table. 

TABLE  40 


Constituents. 

Human  milk. 

Cow's  milk. 

Water                     

88.20 

87.10 

-r,        .      (  Casein  .  . 

0.75 

3  02 

Proteins  j  Albumin 

1  00 

0  53 

Fat  

3.50 

3.69 

Sugar  

6.20 

4.88 

Ash  

0.25 

The  albumin  acts  as  a  protective  colloid  for  the  casein  and  this  ex- 
plains the  different  behavior  of  the  two  substances.  Several  American 
medical  men  have  realized  this  difference  and  favor  the  addition  of 
protective  colloids  such  as  gum  arabic,  dextrin,  etc.,  to  cow's  milk  be- 
fore giving  it  to  children.  Alexander  has  shown  that  cow's  milk  may  be 
made  to  react  much  more  like  human  milk  if  the  protective  colloids  are 
added.  Some  authors  have  also  claimed  that  casein  from  the  two 
sources  is  not  the  same.  This  is  probably  due  to  the  protective  action 
of  the  albumin,  because  by  dissolving  the  casein  from  human  milk,  and 
reprecipitating  several  times,  it  resembles  more  and  more  that  from 
cow's  milk. 


PART  II 
INDUSTRIAL  COLLOIDAL  CHEMISTRY 


PREFACE  TO  PART  II 


THE  chemistry  of  the  colloidal  state  of  matter  has  become  of  such 
great  importance  to  the  chemical  industries  that  it  has  seemed  wise 
to  devote  several  chapters  in  this  book  to  a  side  of  the  question  that 
is  particularly  interesting  to  the  technical  chemist.  As  the  subject 
is  far  too  inexhaustible  to  be  dealt  with  comprehensively  in  the  space 
available  it  has  been  found  necessary  to  omit  any  protracted  discus- 
sion of  the  wide  field  of  biological  chemistry.  It  has  also  been  found 
impossible  to  deal  adequately  with  the  colloidal  chemistry  of  carbo- 
hydrates and  organoplastics,  owing  to  the  fact  that  a  great  deal  of 
the  necessary  information  is  unavailable  in  the  literature. 

ELLWOOD  B.   SPEAR. 


240 


CHAPTER   XIII 

INTRODUCTION  TO  PART  II 

From  a  practical  point  of  view  some  of  the  most  desirable  properties 
of  matter  such  as  plasticity,  elasticity,  etc.,  are  possessed  in  the  highest 
degree  by  amorphous  substances.  As  examples  may  be  cited  rubber, 
leather,  cellulose,  glue,  organoplastics,  and  many  others.  While  all 
amorphous  substances  are  not  colloidal  in  nature  (some  pure  metals 
and  their  alloys  for  instance)  most  of  them  belong  in  the  realm  of  col- 
loidal chemistry.  Hence  many  generalizations  concerning  the  amor- 
phous state  are  applicable  to  the  colloidal.  Some  of  these  fundamental 
ideas  have  been  emphasized  by  Lewis  *  in  an  article  on  the  chemistry  of 
amorphous  solids.  His  basic  idea  in  a  somewhat  modified  form  will  be 
employed  occasionally  in  the  remaining  chapters  of  this  book. 

Colloidal  particles  may  be  conceived  to  be  built  up  of  a  relatively 
small  unit  U  associated  with  itself  by  polymerization,  condensation, 
adsorption  or  otherwise,  n  times.  A  single  particle  would  therefore  be 
represented  by  the  formula  Un.  It  is  not  necessary  at  this  point  to 
make  any  assumption  as  to  the  nature  of  the  association.  It  may  be 
chemical  or  physical,  or  both. 

U  may  be  a  single  molecule  of  the  substance  in  question  as  in  the  case 
of  colloidal  gold  or  other  metals;  or  it  may  be  the  union  of  several 
smaller  units  t/i,  C/2  of  different  substances  as  in  the  case  of  gelatin, 
which  is  a  condensation  product  of  several  amino  acids;  finally  it  may 
be  a  group  that  is  represented  by  the  simplest  possible  formula  under 
the  circumstances.  As  instances  may  be  cited  rubber  CsHg,  or  cellulose 
CeHioOs.  The  formulas  for  rubber  and  cellulose  would  therefore  be 
(CsHs)^  and  (C&RioO^n  respectively,  where  n  may  or  may  not  have  the 
same  value  in  the  two  instances.  N  is  a  pure  number  and  may  have 
many  different  values  in  the  same  colloidal  substance  where  the  parti- 
cles differ  in  size. 

The  point  of  view  as  outlined  above  gives  a  somewhat  clearer  conception 

of  many  phenomena  met  with  in  colloidal  chemistry.     For  instance  we 

are  now  able  to  distinguish  between  two  different  kinds  of  coagulation. 

The  tiny  droplets  of  oil  in  oil  emulsions  on  separating  from  the  water 

flow  together  to  form  a  continuous  phase  oil,  in  which  case  n  becomes 

1  if  oil  exists  in  the  liquid  state  as  molecules.     On  the  other  hand  it  is 

*  Jour.  Soc.  Chem.  Ind.,  35,  12  (1916). 

241 


242  CHEMISTRY  OF  COLLOIDS 

probable  that  in  the  case  of  colloidal  suspensions  of  metals  the  particles 
do  not  flow  together  during  coagulation,  but  on  the  contrary  larger 
entities  are  formed  by  surface  contact  of  the  smaller  particles.  The 
latter  process  could  be  represented  thus :  Unr  +  Un"  .  .  .  =  Un'  Un" 
.  .  .  where  Un'  Un"  ...  is  an  individual  in  a  granular  precipitate. 

Between  these  two  extremes,  liquid  particles  suspended  in  a  medium, 
on  the  one  hand,  and  solid  particles  on  the  other,  all  possible  grades 
exist.  For  instance  colloidal  ferric  hydroxide,  highly  dispersed,  has 
many  properties  of  liquids  in  a  similar  condition.  When  it  is  coagu- 
lated, however,  the  resulting  flocks  cannot  be  considered  a  pure  liquid. 
In  fact  it  is  better  to  call  this  and  other  solids  not  having  a  crystalline; 
structure,  supercooled  liquids. 

Gelatin  solutions  present  a  still  more  complicated  system.  As  long 
as  the  gelatin  is  highly  dispersed  and  the  viscosity  of  the  solution  is  low, 
because  of  the  high  temperature,  the  particles  of  the  disperse  phase, 
gelatin,  may  be  represented  by  the  formula  Un.  If  the  temperature  is 
lowered  and  gelation  takes  place  the  water  becomes  the  disperse  phase, 
and  the  gelatin  is  continuous,  or  nearly  so.  Following  out  the  scheme 
proposed,  we  could  now  say  that  the  globules  of  water  in  the  gelatin 
may  be  represented  by  the  formulas  Un',  Un"  .  .  .  where  U  =  H20. 

The  fundamental  difference  between  the  growth  of  particles  in  Zsig- 
mondy's  nuclear  solutions  and  true  coagulation  becomes  clear  from 
similar  considerations.  As  long  as  gold  is  being  reduced  from  a  com- 
pound the  molecules  .will  be  deposited  upon  the  surface  of  the  particles 
already  present,  and  these  particles  will  increase  in  size.  In  other  words, 
the  values  of  n'}  n",  .  .  .  become  greater.  If  a  precipitating  agent  is 
now  added  the  particles  clot  together  by  surface  contact  and  form  a 
granular  precipitate. 

The  change  of  a  reversible  colloid  to  the  irreversible  state  may  be 
explained  on  the  same  basis.  N  may  or  may  not  be  altered  during  the 
coagulation,  but  as  soon  as  the  changes  in  U  are  sufficiently  fundamental 
the  colloid  becomes  irreversible.  A  very  good  illustration  is  afforded 
by  silicic  acid,  which  becomes  an  irreversible  colloid  as  soon  as  suffi- 
cient water  is  driven  off  to  cause  a  change  in  the  chemical  composition 
of  the  silicic  acid.  Another  very  good  example  is  that  of  caoutchouc  and 
the  finished  product,  rubber.  Both  of  these  substances  are  colloidal, 
but  during  vulcanization  a  fundamental  alteration  has  taken  place  in 
U  because  sulfur  has  been  introduced. 

It  should  be  noted  in  this  connection  that  while  the  change  in  U  in 
rubber  is  probably  of  a  chemical  nature,  many  instances  are  known  in 
which  the  unit  acquires  very  different  properties,  yet  it  is  next  to  im- 
possible to  imagine  that  chemical  reactions  have  taken  place,  e.g.,  the 
addition  of  a  small  quantity  of  gelatin  to  a  colloidal  gold  solution. 


CHAPTER  XIV 
SMOKE,  FLUE  FUMES,  LIQUID  PARTICLES  IN  GASES 

Solid  and  liquid  particles  ejected  from  cement  plants,  smelters,  re- 
finers, etc.,  have  often  been  a  source  of  direct  loss  to  the  companies, 
and  have  in  many  instances  been  so  destructive  to  the  surrounding  prop- 
erty that  a  great  deal  of  litigation,  both  on  the  part  of  private  individuals 
and  of  the  State,  has  resulted.  Until  recent  years  it  was  not  recognized 
that  the  amelioration  of  these  conditions  involved  problems  of  colloidal 
chemistry,  but  according  to  Wo.  Ostwald's  comprehensive  classification  on 
page  (27)  these  systems  fall  under  the  heads  7  and  8,  G  +  S  (gas  +  solid) 
and  G  -f-  L  (gas  +  liquid). 

The  Smoke  Nuisance.  —  The  prevention  of  smoke  in  large  cities, 
especially  from  locomotives  and  manufacturing  establishments,  has 
commanded  considerable  attention.  Any  apparatus  capable  of  pre- 
venting smoke  is  too  cumbersome  for  use  on  a  locomotive.  The  most 
effective  means  is,  of  course,  the  electrification  of  the  railways  within 
the  city  limits;  and  it  is  safe  to  predict  that  the  time  will  come  when 
steam  locomotives  will  not  be  employed  for  city  transportation. 

In  the  case  of  tall  chimneys  the  problem  generally  resolves  itself  into 
one  of  scientific  construction  and  stoking.  It  is  obvious  that  if  all  the 
coal  can  be  burned,  in  the  firebox,  or  the  dust  recovered,  pressed  into 
briquettes  and  finally  burned,  the  result  is  a  net  gain  for  the  company, 
provided  the  cost  of  installation  and  the  operating  expenses  are  not  too 
great.  Where  the  latter  is  the  case  companies  will  naturally  refuse  to  take 
preventive  measures  until  obliged  to  do  so  by  law.  The  plant  neces- 
sary to  eliminate  the  smoke  nuisance  is  similar  to  that  employed  for 
the  prevention  of  flue  fumes  from  blast  furnaces  and  smelters.  It  will 
be  described  under  the  next  heading. 

Flue  Fumes.  —  The  fumes  from  blast  furnaces,  roasters,  converters, 
etc.,  consist  of  gases,  mingled  with  liquid  and  solid  particles,  and  are 
often  of  considerable  economic  value.  Especially  where  sulfur  dioxide, 
sulfur  trioxide,  copper,  zinc  and  lead  salts  are  ejected  from  the  plant, 
the  fumes  are  deleterious  to  the  surrounding  property;  and  many  of 
these  plants  have  been  obliged  by  law  to  condense  the  larger  portion 
of  the  fumes  causing  the  damage.  Moreover  where  the  gases  from 
blast  furnaces  are  to  be  used  for  the  production  of  power  in  internal 

243 


244  CHEMISTRY  OF  COLLOIDS 

combustion  engines,  the  larger  dust  particles  must  be  eliminated. 
Several  of  the  devices  in  operation  will  be  briefly  treated  in  this  chapter, 
but  only  where  distinct  principles  of  colloidal  chemistry  are  involved. 
For  details  of  construction  and  of  operation  recourse  must  be  had  to 
works  on  metallurgy,*  or  to  the  original  articles  cited.  The  methods 
employed  may  be  described  as  washing,  centrifugalizing,  settling, 
arresting,  filtering,  and  electrical  precipitation.  In  all  these  cases  U 
is  probably  not  materially  altered.  On  the  other  hand,  n  may  be 
increased  very  greatly,  or  possibly  the  particles  may  join  together  by 
surface  contact  to  form  irregular  masses  as  suggested  in  the  introduc- 
tion to  Part  II. 

Washing.  —  Washing  the  gases  is  usually  accomplished  by  means 
of  fine  sprays,  by  bubbling  through  water,  or  churning  in  agitators. 
This  method  is  employed  for  preparing  gases  of  blast  furnaces  for  use 
in  internal  combustion  engines.  The  resulting  liquid  is  very  corrosive 
and  soon  affects  the  retainers.  In  short  it  is  impractical  for  use  in 
large  plants  where  several  hundred  thousand  cubic  feet  of  gas  per 
minute  pass  through  at  a  temperature  sometimes  varying  from  100  to 
400°  C. 

Centrifugalizing.  —  Here  the  gases  are  either  passed  tangentially 
into  a  cylindrical,  stationary  container  where  the  particles  are  thrown 
against  the  sides. and  fall  down;  or  the  gases  are  passed  axially  into  a 
rapidly  rotating  cylindrical  shell  provided  with  baffle  plates.  The  first 
of  these  methods  is  not  efficient  when  the  particles  are  very  fine,  and  the 
second  involves  too  much  machinery  for  large  volumes  of  gases. 

Settling  Chambers.  —  That  dust  particles  will  settle  out  of  still  gases 
better  than  they  will  from  those  that  are  moving  rapidly,  is  a  matter  of 
common  experience.  Every  housewife  knows  that  dust  blows  in  the 
doors  and  windows,  but  settles  out  on  the  furniture  of  the  room.  The 
cause  of  this  lies  in  the  fact  that  the  dust  particles  cannot  remain  sus- 
pended in  the  air  as  soon  as  the  velocity  of  the  latter  is  sufficiently 
reduced.  Advantage  has  been  taken  of  this  property  of  suspended 
particles  to  recover  valuable  material  from  the  fumes  at  the  Cananea,f 
Copper  Queen,  J  and  Anaconda  §  copper  works.  The  gases  are  conducted 
through  chambers  and  galleries  where  the  velocity  is  reduced  to  the 
necessary  rate  1f  before  they  pass  out  through  the  large  stack. 

*  Hofman,  Metallurgy  of  Copper,  page  218;  Hofman,  General  Metallurgy,  page 
831. 

t  Shelby,  Eng.  Min.  J.,   86,  204  (1908). 
t  Lee,  Eng.  Min.  J.,  90,  504  (1910). 
§  McDougal,  Canad.  Min.  Rev.,  24,  26  (1905). 

Austin,  Tr.  A.I.M.E.,  37,  478  (1906). 
If  Kiddie,  Tr.  A.I.M.E.,  40,  900  (1909). 


SMOKE,  FUEL  FUMES,  LIQUID  PARTICLES  IN  GASES        245 


The  researches  of  Kiddie,* 
Leef  and  others  have  shown 
that  the  temperature  must  be 
below  300°  C.  and  the  velocity 
reduced  to  about  200  ft.  per 
minute  before  the  very  small 
particles  will  fall  out.  In  the 
downcomers  and  in  some  other 
parts  of  the  plant  the  velocity 
may  be  as  high  as  1500  ft.  per 
minute,  hence  it  is  obvious  that 
enormous  spaces  must  be  erected 
for  settling  chambers.  Fig.  34 
is  a  diagram  of  the  flues  at 
Washoe  smeltry,  Anaconda, 
where  some  of  the  fumes  travel 
four-fifths  of  a  mile  before  they 
are  allowed  to  pass  into  the  final 
stack.  Fig.  35  is  a  transverse 
section  of  the  twin  settling 
chamber.  Over  80  tons  of  fume 
are  collected  every  24  hours  by 
this  plant.  Particles  that  pass  FIG.  34.  Plan  of  flues  at  Washoe  smeltry, 
into  the  open  from  the  plant  are  Anaconda, 

sufficiently  fine   to   go   through  A  is  the  stack,  B  the  twin  settling  chamber, 

a  sieve  of  200  mesh.     (Figs.  34      c, th/  ^  furnace  Plant;  ^  the/°fsler 

plant,  E  the  converter  plant,  and  F  the 

and  35.)  reverberatory  plant. 


Grade 


A  A 

FIG.  35.    Transverse  section  of  the  twin  settling  chamber  at  the  Washoe  smeltry, 
Anaconda.    A,  A  are  the  tunnels  where  the  precipitated  fume  is  loaded  into  cars. 

Arresting  by  Means  of  Baffle  Plates  or  Wires.  —  The  efficiency  of 
the  settling  chamber  is  greatly  increased  by  the  installation  of  baffle 


*  Lc. 


Me. 


246 


CHEMISTRY  OF  COLLOIDS 


plates  or  wires  to  arrest  the  progress  of  the  dust.  When  the  particles 
are  brought  into  contact  with  one  another  on  some  surface,  they  form 
clumps  that  are  not  easily  disintegrated.  These  clumps  can  therefore 
be  shaken  off  the  baffles  by  hand  or  by  machinery  without  causing  the 
particles  to  enter  the  air  current  again. 


100 


100 


150  200 

^Length  of  Mue  in  feet 
FIG.  36. 


250 


The  Roesing  wire  system  *  is  in  successful  operation  at  Great  Falls,  f 
Before  the  present  system  was  adopted,  elaborate  experiments  under 
actual  working  conditions  were  carried  out  with  the  various  arresting 
devices.    The  results  are  shown  graphically  in  Fig.  36.     Curves  32  and 
34  show  that  only  30  to  40  per  cent  of  the  dust  was  collected  when  the 
gases  passed  through  the  open  or  ordinary  flue.     Freudenburg  plates, 
*  Hofman,  General  Metallurgy,  p.  846  (1913). 
t  Herrick,  Mines  and  Minerals,  30,  257  (1909). 
Goodale,  Tr.  A.I.M.E.,  40,  891  (1909). 


SMOKE/  FLUE  FUMES,  LIQUID  PARTICLES  IN  GASES        247 

suspended  in  such  a  manner  that  the  gas  current  strikes  the  narrow 
edge,*  increase  the  efficiency  somewhat  as  shown  by  curve  33.  When 
baffles,  3|  in.  wide,  are  hung  so  that  the  gas  strikes  the  side  of  the 
plates,  the  efficiency  is  70  per  cent,  as  shown  by  curve  36.  Wider  plates, 
6J  in.,  curve  35,  arrest  still  more  of  the  dust;  but  the  cross-sectional 
area  is  reduced  50  per  cent,  and  as  a  consequence  the  draft  is  very  greatly 
interfered  with.  Wire  baffles  give  a  high  efficiency,  and  do  not  seri- 
ously obstruct  the  draft.  There  are  about  1,200,000  steel  wires  in  the 
system,  placed  2.3  in.  apart  from  center  to  center,  and  divided  into 
two  sets.  One  set  is  made  up  of  No.  8W.  and  M.  wires,  each  16  ft. 
long,  while  the  other  is  composed  of  No.  10  wire,  20  ft.  long. 

Filtering.  —  Filtering  by  means  of  woolen  or  cotton  bags  is  another 
.device  employed  to  condense  fume  in  the  Mammoth  Smeltery,  Kennett, 
Calf  Air  is  admitted  to  cool  the  gases  to  100  degrees  or  less,  and  finely 
divided  zinc  oxide,  or  calcium  hydroxide,  is  blown  in  to  neutralize  the 
SOS.  The  neutralization  is  necessary  or  the  bags  would  be  destroyed 
within  a  few  hours.  The  bag  house  is  210  ft.  long,  63  ft.  wide,  and 
contains  3000  woolen  or  cotton  bags,  each  34  ft.  long  and  18  in.  in  di- 
ameter. About  10  tons  of  fume  are  collected  every  twenty-four  hours 
by  this  means. 

Electrical  Precipitation,  Cottrell  Process.  —  The  removal  of  sus- 
pended particles  from  gases  by  aid  of  electric  discharge  was  suggested 
by  Hohlf eld  t  as  a  means  of  preventing  the  smoke  nuisance,  as  early  as 
1824.  While  many  attempts  have  since  been  made  to  render  the 
method  commercially  useful,  the  efforts  of  Cottrell  were  the  first  to  be 
crowned  with  success.  The  development  of  the  process  is  interest- 
ingly told  by  the  inventor  in  some  of  his  publications.  §  The  principles 
involved  are  more  or  less  familiar  to  every  physicist,  and  consist  in 
bringing  the  particles  in  contact  with  fine  points  having  a  high  elec- 
trical potential.  The  particles  thus  become  charged  with  the  same 
sign  as  the  fine  points,  and  are  attracted  to  and  discharged  upon  a 
large  plate  electrode  having  the  opposite  sign,  and  a  low  potential. 
The  precipitated  material  may  be  easily  washed  or  shaken  off  the 

*  Hofman,  General  Metallurgy,  p.  845  (1913). 
t  Campbell,  Min.  Sc.  Press,  96,  30  (1908). 

Martin,  Min.  Eng.  World,  29,  309  (1908). 

Rice,  Eng.  Min.  J.,  90,  614  (1911). 

Martin,  Mines  and  Minerals,  33,  323  (1913). 
J  Kastner  Archiv.  Naturl.,  2,  205-6  (1824). 
§  U.  S.  Pats.  866843,  895729,  945917,  1016476,  1035422,  1067974. 

Jour.  Ind.  Eng.  Chem.,  3,  542  (1911). 

Smithsonian  Report,  p.  653  (1913). 

Mining  Industry,  vol.  XXIII,  p.  889  (1914). 


248 


CHEMISTRY  OF   COLLOIDS 


SMOKE,  FLUE  FUMES,  LIQUID  PARTICLES  IN  GASES        249 

plate  electrodes,  which  in  practice  are  usually  lead  or  iron.  The  high 
potential  electrodes  are  made  of  mica  or  asbestos  and  the  fibers  of  the 
material  serve  as  the  necessary  fine  points.  Direct  current  must  be 


SECTION  THROUGH 
PRECIPITATING  UNIT 
BALAKLALA  CON.  COPPER  CO 
CORAM,  CAL. 


FIG.  38. 


employed,  and  is  obtained  from  an  alternating  current  by  means  of 
rectifiers.  The  potential  of  the  asbestos  or  mica  electrodes  is  in  some 
plants  raised  as  high  as  100,000  volts.  The  electrical  method  of  pre- 
cipitation is  particularly  adapted  for  use  in  plants  where  sulfuric  acid 


250  CHEMISTRY  OF  COLLOIDS 

particles  are  present  in  the  fume.  The  acid  recovered  often  pays  the 
cost  of  operation  several  times  over. 

The  electrical  method  was  installed  in  the  Balaklala  Copper  Works, 
Coram,  Cal.,  after  many  tests  had  been  made  with  other  methods  of 
precipitation.  Fig.  37*  is  a  plan  of  the  nine  electrical  precipitation 
units,  or  chambers,  in  their  relation  to  the  flue  system  and  stack.  The 
large  fans  are  not  necessary  for  the  operation  of  the  precipitation  sys- 
tem, but  are  employed  to  dilute  the  flue  gases  with  air  in  order  to  reduce 
the  concentration  of  the  sulfur  dioxide  below  the  maximum  allowed 
by  law,  viz.,  three-fourths  of  one  per  cent.  Fig.  38  shows  a  cross 
section  through  one  of  these  units.  The  double  lines  represent  the  col- 
lecting electrodes,  each  6  inches  wide  and  10  ft.  long,  made  of  No.  10 
sheet  iron.  The  dotted  lines  are  the  high  potential  electrodes.  200,000 
to  300,000  cubic  of  gas  at  a  temperature  of  100  to  150°  C.  pass  through 
this  system  per  minute  under  normal  working  conditions,  and  6  to  8 
tons  of  precipitated  material  are  collected  every  24  hours. 

The  electrical  process  has  been  installed  by  the  Riverside  Portland 
Cement  Co.  near  Riverside,  Cal.,  where  nearly  100  tons  of  dust  are  col- 
lected every  24  hours.  The  dust  contains  considerable  K20  which  is 
worked  up  into  valuable  fertilizer.  The  process  is  employed  in  many 
silver,  zinc,  and  arsenic  plants,  and  is  working  successfully  at  Catasauqua, 
Pa.,  for  the  recovery  of  volatilized  potash  from  feldspar.  At  the  North 
Works  of  the  American  Steel  and  Wire  Company,  Worcester,  Mass.,  it 
is  employed  for  collecting  volatilized  hydrochloric  acid.  An  electrical 
process  based  on  the  same  principles  is  controlled  for  the  State  of  Cali- 
fornia by  the  Petroleum  Rectifying  Company,!  San  Francisco,  for  the 
dehydration  of  crude  petroleum.  The  oil  and  water  form  an  emulsion 
sometimes  very  difficult  to  separate  into  two  layers. 

*  Figs.  37  and  38  are  taken  from  an  article  by  F.  G.  Cottrell  in  the  Jour.  Indus, 
and  Engineering  Chem.,  1  (1911). 
t  Oil  Age,  April  21  (1911). 


CHAPTER  XV 
RUBBER 

The  study  of  the  physical  and  chemical  properties  of  rubber  has 
received  a  decided  impetus  as  a  result  of  recent  developments  in  col- 
loidal chemistry,  for  it  cannot  be  denied  that  in  caoutchouc,  the  fun- 
damental substance  from  which  commercial  rubber  is  made,  we  have  a 
typical  colloidal  body.  Many  of  the  processes  in  the  manufacture  of 
both  the  crude  rubber,  caoutchouc,  and  the  finished  product,  rubber, 
such  as  the  coagulation  of  the  latex,  find  no  explanation  from  the  purely 
crystalloidal  chemical  standpoint.  Unfortunately  in  the  discussions 
on  the  subject  it  has  not  always  been  recognized  that  in  most  instances 
both  the  colloidal  and  crystalloidal  processes  take  place  simultaneously. 
Consequently  important  facts  are  often  ignored  by  the  extreme  advo- 
cates of  the  colloidal  and  the  purely  chemical  schools.  Only  by  a  proper 
perspective  involving  both  views,  can  we  arrive  at  the  true  explanation 
of  many  of  the  phenomena  connected  with  the  chemistry  of  rubber. 

The  Latex 

The  latex  from  which  caoutchouc  is  obtained  is  a  milk-like  fluid  dif- 
fering somewhat  in  its  properties  according  to  its  origin.  Biologically 
it  is  the  sap  of  certain  trees  or  shrubs;  chemically  it  is  a  disperse  system 
consisting  of  globules  of  caoutchouc  suspended  in  a  watery  liquid. 

A  chemical  analysis  of  the  latex  from  Funtumia  elastica  gave  the 
following  results : 

Per  cent 

Water  (reacts  acid) 56.9 

Caoutchouc 36. 53 

Resins 4. 16 

Protein  and  minerals 2 . 88 

The  simplest  formula  that  we  can  give  to  caoutchouc  is  Un,  where 
U  =  CsHg.  Many  chemists  believe  that  U  should  be  given  the  value 
CioHie.  In  either  case  caoutchouc  may  be  considered  as  a  conden- 
sation product  of  isoprene,  C£H8. 

As  might  be  expected  from  the  standpoint  of  colloidal  chemistry, 
molecular  weight  determinations  of  caoutchouc  in  solvents  give  unsatis- 
factory results.  This  is  accounted  for  by  the  fact  that  the  average  n 
is  large,  and  the  osmotic  pressure  correspondingly  low.  Therefore  the 
lowering  of  the  freezing  point  becomes  negligible. 

251 


252  CHEMISTRY  OF   COLLOIDS 

Wo.  Ostwald,  E.  Fickendey,  V.  Henri  and  others  have  classified  the 
latex  as  an  emulsion,  class  5,  page  28.  On  the  other  hand  it  is  strenuously 
contended  by  Ditmar  *  that  the  globules  are  solid  particles,  and  the  latex 
must  therefore  be  considered  a  coarse  suspension,  class  4.  C.  0.  Weber  f 
considers  the  globules  are  composed  of  liquid  terpenes  surrounded  by  a 
protective  layer  of  protein  (Hevea  braziliensis),  or  peptone  (Funtumia). 
Other  authors,  notably  J.  Henri  J  and  E.  Fickendey,  §  accept  the  idea  of 
a  protective  layer,  but  believe  that  the  substance  of  the  globules  is 
caoutchouc.  The  last  named  author  regards  the  caoutchouc  in  the 
globules  as  a  liquid,  claiming  that  it  becomes  a  solid  only  on  coagulation. 
Whether  the  latex  is  to  be  regarded  as  a  suspension  or  an  emulsion  is 
not  of  paramount  interest  to  the  industrial  chemist,  however. 

Charge  and  Brownian  Movement.  —  The  particles  or  globules  in  the 
latex  are  charged  negatively  and  are  in  rapid  motion.  This  phenom- 
enon is  termed  " Brownian  Movement."  See  page  40. 

Source.  —  The  trees,  shrubs  or  vines  that  yield  the  latex  are  found 
in  warm  climates  under  the  most  varying  conditions  of  soil  and  mois- 
ture, and  belong  to  the  botanical  orders,  Euphorbiaceae,  Apocynaceae, 
Urticacese,  Compositse.  Para  rubber  comes  from  Hevea  braziliensis, 
formerly  called  Siphonia  braziliensis,  which  grows  wild  in  Brazil  in  hot 
moist  climates,  and  is  now  cultivated  on  rubber  plantations  in  Asia, 
notably  Ceylon,  and  in  the  -Malaya  Peninsula.  Ceara  or  Manicola 
rubber  is  obtained  from  Manihot  Glaziovii,  a  tree  that  grows  on  dry 
rocky  soil  in  South  America.  "Caucho"  rubber  comes  from  Castilloa 
elastica  which  grows  in  Peru.  The  same  tree  in  Central  America  yields 
"Ule"  rubber.  Guayule  rubber  is  obtained  from  the  Guayule  shrub 
which  grows  in  Mexico  under  almost  desert  conditions.  In  Africa  the 
chief  sources  of  native  rubber  are  Funtumia  elastica,  or  Kickxia,  ar>d 
the  Landolphia  vines.  In  Asia  rubber  is  obtained  from  a  wild  tree, 
Ficus  elastica,  in  addition  to  the  many  cultivated  varieties  of  rubber 
producing  plants. 

Size  of  the  Particles.  —  The  suspended  globules  of  caoutchouc 
found  in  the  latex  vary  in  size  within  comparatively  wide  limits.  Those 
of  Hevea  have  been  found  by  Fickendey  to  have  a  diameter  of  0.5-2.5/z. 
On  the  other  hand,  few  particles  in  the  latex  of  Funtumia  have  a  di- 
ameter as  great  as  O.Iju,  and  the  milk  appears  almost  homogeneous 
under  the  most  powerful  microscope. 

*  Der  Kautschuk,  p.  23  (1912). 
t  Gummi  Zeit.,  No.  7  (1902). 

Her.  d.  Deutsch.  chem.  Ges.,  3108  (1903). 
J  Le  Caoutchouc  et  la  Guttapercha,  No.  27  (1906). 
§  Kobbidzeitschr.,  8,  43-7  (1911). 


RUBBER  253 

Coagulation  of  the  Caoutchouc. 

The  methods  in  practice  for  the  separation  of  the  caoutchouc  from 
the  watery  liquid  are:  — 

1.  Heat  and  smoking:   employed  by  the  natives  for  preparing  Para 
rubber. 

2.  Boiling:  Funtumia. 

3.  Evaporation:    drying  on  the  bark  as  it  runs  down  the  wound, 
whereby  elastic  strings  called  "scrap"  are  obtained;    or,  pouring  the 
milk  into  saucer-shaped  stumps  where  the  water  is  either  soaked  up  by 
the  wood  or  evaporated  into  the  air.     The  masses  of  rubber  so  obtained 
are  called  "cakes." 

4.  Dilution  with  water. 

5.  Addition  of  precipitating  agents:    acids,  alum,  etc.,  are  usually 
employed  on  rubber  plantations. 

One  and  the  same  method  of  coagulation  is  not  always  applicable  to 
the  latex  from  different  sources,  consequently  in  practice  that  method 
must  be  sought  out  which  will  produce  the  best  caoutchouc  from  a 
given  latex.  The  problem  of  choosing  the  most  efficacious  method  is 
further  complicated,  however,  by  the  presence  in  the  latex  of  either 
protein  or  peptone,  both  of  which  act  as  protective  colloids  for  the  glo- 
bules. Moreover  it  is  stated  by  some  experimenters  that  the  compo- 
sition of  the  latex  varies  with  the  season  of  year  and  even  with  the  height 
from  the  ground  of  the  incision  in  the  bark.  On  the  other  hand  J.  v. 
Wiesner  has  found  that  the  resin  in  the  latex  of  the  euphorbiacese  is 
independent  of  the  geographical  distribution. 

The  objections  to  be  raised  against  the  first  and  third  methods,  viz, 
heating  and  smoking,  and  evaporation,  are  that  many  of  the  unde- 
sirable constituents  such  as  dirt,  bark,  resin,  protective  colloids,  etc., 
are  left  in  the  coagulant,  caoutchouc.  Some  of  these  substances  can 
be  removed,  of  course,  by  subsequent  treatment  such  as  grinding  and 
washing,  drying  in  air  or  vacuo,  but  mineral  matter,  resins,  and  proteins 
still  remain  and  cannot  be  eliminated  except  by  expensive  processes. 
It  must  be  confessed,  however,  that  Para  rubber  obtained  by  the  natives 
of  Brazil  is  superior  to  that  made  by  the  most  scientific  methods  yet 
devised  on  plantations.  It  is  a  matter  not  yet  decided  whether  the 
superiority  of  Para  rubber  is  due  to  the  method,  or  to  the  inherent 
qualities  of  the  latex. 

According  to  the  researches  of  E.  Fickendey  all  varieties  of  latex  may 
be  coagulated  by  boiling.  This  operation  destroys  the  protective  colloids 
and  the  coagulation  is  then  caused,  or  at  least  aided,  by  the  electrolytes 
present  in  the  natural  milk. 


254  CHEMISTRY  OF   COLLOIDS 

To  V.  Henri,  and  later  to  E.  Fickendey,  we  are  especially  indebted 
for  a  clear  presentation  of  the  process  of  coagulation  and  of  the  effect 
of  precipitating  agents.  According  to  these  investigators  the  process 
takes  place  in  steps,  and  may  be  followed  to  some  extent  under  the  mi- 
croscope. During  the  first  step,  depending  upon  the  method  employed, 
the  globules  may  gather  together  and  form  clumps,  where  the  particles 
touch  one  another  but  do  not  flow  into  a  compact,  elastic  mass;  or  long 
filaments  may  be  formed.  The  clumps,  or  flocks,  may  remain  sus- 
pended in  the  liquid  or  may  sink  to  the  bottom  and  finally  form  a 
powder.  Mechanical  agitation  will  merely  subdivide  the  flocks,  and 
the  process  therefore  resembles  the  coagulation  of  blood.  Henri  calls 
this  phenomenon  agglutination.  In  case  long  fibers  are  formed  a  further 
action  takes  place  during  which  the  particles  flow  together  and  form  an 
elastic  net  work.  The  formation  of  an  elastic  mass  from  the  net  work 
is  accelerated  by  boiling,  or  by  mechanical  manipulation. 

There  is  a  want  of  conformity  of  opinion  among  different  investi- 
gators with  regard  to  what  actually  happens  during  the  last  step.  If 
the  particles  are  composed  of  liquid  caoutchouc  surrounded  by  a  film 
of  protective  colloid,  then  we  must  conclude  that  a  solidification  occurs 
as  well  as  coagulation.  Some  writers  are  inclined  to  the  belief  that  a 
polymerization  takes  place  whereby  larger  molecules  are  formed.  Ac- 
cording to  E.  Fickendey  this  is  a  pure  but  not  impossible  assumption 
for  which  we  have  no  direct  experimental  proof. 

Effect  of  Precipitating  Agents.  —  In  practice  we  are  interested  in  the 
effect  of  precipitating  agents  on  the  latex  as  it  comes  from  the  tree,  but 
in  order  to  find  the  scientific  basis  for  this  effect  it  is  necessary  to  elimi- 
nate all  the  dissolved  salts  already  present  in  the  natural  milk.  This 
was  effected  by  Henri  on  the  latex  from  Hevea  braziliensis  by  means  of 
dialysis  through  collodion  filters,  see  page  36.  His  results  may  be 
summarized  as  follows: 

1.  Alcohols  were  without  effect.     Up  to  this  time  alcohols  had  been 
considered  universal  precipitants  for  caoutchouc,  but  we  can  now  safely 
conclude  that  the  precipitation  is  due  to  the  combined  effect  of  alcohol 
plus  the  dissolved  electrolytes. 

2.  Salts  of  the  alkaline  metals  likewise  cause  no  precipitation. 

3.  Salts  of  the  alkaline  earth  cause  formation  of  flocks. 

4.  Salts  of  the  heavy  metals  at  a  much  smaller  concentration  cause 
formation  of  flocks.     In  none  of  these  cases  however  is  an  elastic  mass 
formed. 

5.  Alkalis  have  little  effect. 

6.  Acids  cause  the  formation  of  flocks.     HC1,   HNOs,  Hac,   have 
approximately  the  same  effect.     H2S04  is  much  more  efficacious  and 


RUBBER  255 

very  dilute   trichloracetic   acid   causes   the    separation   of    an    elastic 
mass. 

8.  Alcohol  +  salts  of  univalent  metals  at  high  concentration  cause 
flock  formation. 

9.  Alcohol  +  acids,  or  salts  of  bivalent  or  trivalent  metals,  give  an 
elastic  mass. 

10.  Alcohol  +  alkalis  do  not  cause  coagulation,  but  on  the  contrary 
retard  the  action  of  other  precipitants. 

It  will  be  noticed  that  the  behavior  on  the  part  of  precipitating  agents 
is  analogous  to  the  effect  on  protein  (see  page  216).  While  the  evidence 
is  not  conclusive,  it  is  nevertheless  probable  from  our  knowledge  of  pro- 
tective colloids,  that  the  protein  film  on  the  outside  of  the  globules  must 
be  destroyed  before  an  elastic  mass  can  be  formed.  Any  solvent  or 
precipitant  for  protein  should,  therefore,  leave  the  globules  more  or 
less  free  to  unite  after  their  electric  charges  have  been  neutralized  by 
an  electrolyte.  The  salts  of  the  heavy  metals  coagulate  protein.  This 
accounts  for  their  precipitating  action  on  the  latex.  This  point  of  view 
is  further  strengthened  by  a  research  on  Kickxia  latex  by  E.  Fickendey.* 
It  was  known  that  this  latex  would  not  respond  to  the  usual  precipitants. 
The  cause  of  this  peculiar  behavior  was  found  to  lie  in  the  fact  that 
Kickxia  latex  contained  peptones  but  no  proteins.  Precipitating  agents 
for  proteins  would,  therefore,  have  no  effect,  while  those  for  peptones, 
such  as  tannin,  would  cause  coagulation. 

D.  Spence  f  has  investigated  the  effect  of  precipitating  agents  on  the 
latex,  Funtumia  elastica,  in  the  natural  state.  His  results  are  in  keep- 
ing with  those  of  Henri  except,  of  course,  that  the  latter  worked  with 
the  dialyzed  latex. 

Crossley  has  found  that  the  acid  used  for  coagulation  is  adsorbed 
according  to  the  laws  that  usually  obtain  for  colloids  (see  page  52). 
Hence  some  of  the  acid  should  be  adsorbed  on  the  globules  and  carried 
down  in  the  elastic  mass  from  which  it  could  be  removed  only  with 
great  difficulty.  This  is  in  agreement  with  facts. 

According  to  the  ideas  laid  down  in  the  introduction  to  Part  II,  the 
coagulation  of  the  latex  may  be  explained  as  follows:  Precipitating 
agents  that  do  not  dissolve  or  coagulate  the  protective  colloid  may 
cause  the  flock  formation  because  the  electric  charges  are  neutralized 
and  the  particles  join  by  surface  contact  only.  Thus  no  increase  of 
n  is  involved.  On  the  contrary  precipitating  mixtures  that  destroy 
the  effect  of  the  protective  colloid  and  neutralize  the  electric  charges 
at  the  same  time  allow  the  particles  to  flow  together,  whereby  n  is 

*  E.  Fickendey,  I.  c. 

t  Liverpool  University.     Institute  of  Commercial  Research  in  the  Tropics. 


256  CHEMISTRY  OF   COLLOIDS 

greatly  increased  and  an  elastic  mass  or  net  work  results.  It  is  also 
possible  that  during  the  formation  of  the  solid  from  the  liquid  caout- 
chouc some  change  of  U  takes  place,  but  this  is  not  necessarily  true. 

Vulcanization 

When  raw  caoutchouc  is  mixed  with  sulfur  and  the  temperature 
raised  sufficiently  a  remarkable  change  of  chemical  and  physical  prop- 
erties takes  place.  The  mass  loses  its  adhesiveness,  called  " tackiness" 
in  practice;  the  elasticity  may  vary  between  great  extremes;  differences 
of  temperature  over  a  comparatively  wide  range  have  little  effect;  it 
is  rendered  insoluble  in  any  liquid  that  does  not  permanently  destroy 
it;  and  finally  it  is  much  more  resistant  to  oxidation,  and  therefore  less 
liable  to  " perish."  The  process  is  known  as  the  "hot  cure"  or  hot  vul- 
canization. Similar  alterations  in  properties,  differing  only  in  degree,  may 
be  brought  about  by  what  is  termed  the  "cold  cure,"  or  cold  vulcaniza- 
tion. The  hot  cure  is  much  more  widely  applied  in  practice. 

Cold  Vulcanization.  —  The  material  in  thin  strips,  or  sheets,  is  passed 
through,  or  dipped  into  a  solution  of  sulfur  chloride  (S2C12)  in  carbon 
disulfide,  or  carbon  tetrachloride.  Some  of  the  sulfur  chloride  re- 
mains united,  physically  or  chemically,  with  the  caoutchouc,  and  the 
excess  is  neutralized  with  ammonia.  By  this  process  the  goods  acquire 
a  soft  velvet  feel,  but  unfortunately  are  liable  to  "perish"  rather  easily. 
A  variation  of  the  method,  technically  known  as  the  vapor  "cure," 
consists  of  treating  the  goods  with  the  vapors  of  sulfur  chloride.  Sub- 
sequent neutralization  with  ammonia  is,  of  course,  necessary. 

Hot  Vulcanization.  —  This  process  is  carried  out  in  one  of  three  ways : 

1.  By  steam  heat  where  the  sheets  are  wound  on  a  drum,  covered 
with  a  steam-proof  wrapper,  and  subjected  to  a  steam   pressure   of 
several  atmospheres. 

2.  Where  massive  articles  are  to  be  vulcanized  a  hot  press,  or  mould, 
is  found  most  convenient.     This  is  termed  the  "press  cure"  and  is  the 
usual  method  employed  in  the  manufacture  and  repairing  of  pneumatic 
tires.     In  both  these  methods  accelerators  are  often  added  together 
with  a  varying  amount  of  "fillers."     Litharge  is  frequently  used  as  an 
accelerator  for  black  goods,  and  lime  or  magnesia  for  white.     Litharge 
cannot  be  employed  for  white  goods  because  of  the  lead  sulfide  formed 
during  the  heating. 

3.  Boots,  shoes,  coats,  etc.,  are  usually  vulcanized  by  the  "dry  heat 
cure."     Here  the  goods  are  hung  in  an  air-tight  chamber  heated  by 
steam.     Litharge  or  other  accelerators  are  necessary  to  accomplish  the 
purpose  in  a  reasonable  length  of  time  —  six  to  seven  hours. 

In  order  to  produce  a  soft  rubber  the  amount  of  sulfur  added  may 


RUBBER  257 

vary  from  2  to  10  per  cent.  Of  this  amount  usually  not  more  than  3 
per  cent  becomes  so  fixed  that  it  cannot  be  extracted  by  the  use  of  such 
solvents  as  acetone.  This  fixed  sulfur  is  generally  termed  "  com- 
bined." Under  ordinary  conditions  in  practice,  regardless  of  the 
amount  added,  all  the  sulfur  does  not  become  " combined,"  but  a 
portion,  varying  with  the  temperature  and  the  duration  of  the  heat 
treatment,  always  remains  "free,"  i.e.,  can  be  removed  by  solvents. 
For  the  production  of  hard  rubber,  or  vulcanite,  as  much  as  30  to  35 
per  cent  of  sulfur  may  be  added,  but  it  seems  to  be  the  general  consensus 
of  opinion  among  chemists  that  not  more  than  32  per  cent  ever  be- 
comes combined.  If  the  amount  of  free  sulfur  is  too  great,  a  portion 
of  it  crystallizes  out  on  the  surface  in  the  form  of  a  pale  yellow  deposit. 
This  phenomenon  is  known  as  "blooming."  In  order  to  prevent  the 
crystallization  of  the  sulfur  on  the  outside  of  certain  goods  the  amount 
of  free  sulfur  must  be  kept  low. 

The  amount  of  sulfur  necessary  to  give  certain  properties,  such  as 
a  desired  elasticity,  depends  a  great  deal  upon  the  physical  state  of  the 
caoutchouc.  In  general  if  the  average  value  of  n  is  lowered,  b}'  mastication, 
for  example,  more  sulfur  will  be  necessary  to  produce  a  desired  result. 

No  substance  has  as  yet  been  found  that  can  adequately  replace  sul- 
fur in  the  hot  cure.  Selenium  does  this  only  to  a  very  limited  extent 
and  the  result  is  not  at  all  satisfactory.  In  the  case  of  the  cold  cure 
some  success  has  been  obtained  by  the  use  of  bromine  in  carbon  disul- 
fide.*  Hypochlorites  and  hypochlorous  acid  have  also  been  used. 

Theories  of  Vulcanization 

A  great  deal  of  work  has  been  done  of  recent  years  on  the  process  of 
vulcanization.  Unfortunately  the  results  of  one  experimenter  often 
seem  to  contradict  those  of  another;  consequently  we  have  few  undis- 
puted facts  upon  which  to  base  our  theories.  This  want  of  accord 
among  the  experimental  results  is  probably  not  due  so  much  to  inaccu- 
rate work  as  it  is  to  the  fact  that  raw  caoutchouc  is  a  complex  product, 
varying  in  properties  to  a  considerable  degree  with  its  source,  method 
of  preparation,  age,  etc.  A  short  summary  and  not  an  extended  dis- 
cussion of  the  two  principal  theories  will  be  given  here.  For  a  more 
comprehensive  review  of  the  subject  the  reader  is  referred  to  the  original 
literature,  or  to  "Der  Kautschuk"  by  Ditmar. 

The  Chemical  Theory.  — ;  The  first  to  suggest  this  theory  as  an  ex- 
planation of  his  own  experimental  results  was  C.  0.  Weber,  f  His 
conclusions  may  be  briefly  stated  as  follows: 

*  C.  O.  Weber,  Chemistry  of  India  Rubber  (1906). 

t  Zeit.  f.  angew.  Chemie,  112,  142  (1894). 


258  CHEMISTRY  OF  COLLOIDS 

1.  Caoutchouc  unites  with  sulfur  without  the  evolution  of  an  appre- 
ciable amount  of  hydrogen  sulfide.     The  reaction  is  therefore  an  addi- 
tion process. 

2.  A  continuous  series  of  compounds  is  formed,  the  highest  member  of 
which  probably  has  the  formula  (CioHi6)ioS2o,  while  the  lowest  may  be 
represented  by  (CioHi6)ioS.    Which  member  of  the  series  is  formed 
depends  upon  the  amount  of  sulfur  added,  the  temperature  and  the 
duration  of  the  treatment. 

3.  The  physical  state  of  the  raw  material  does  not  determine  the 
end-point,  but  may  affect  the  time  and  temperature  factors. 

4.  During  cold  vulcanization  with  S2Cl2  a  similar  series  is  formed. 
The  highest  member  of  this  series  would  therefore  have  the  formula 
(CioHi6)io(S2Cl2)io,  and  the  lowest  (CioHi6)ioS2Cl2. 

The  above  theory  has  been  extended  by  Ditmar  to  include  the  work 
of  Harries  and  Hiibener.*  According  to  this  explanation  the  mole- 
cule of  caoutchouc  has  the  following  formula. 

CH  =  C  -  CH3 
I  I 

CH2     CH2 

I  I 


I  I 

CH3  -  C  =     CH 

Ten  of  these  groups  or  molecules  are  loosely  united  by  the  dotted  lines 
into  a  huge  ring-formed  entity.  Sulfur  or  sulfur  chloride  may  be  added 
to  the  caoutchouc  at  any  one  or  more  pairs  of  double  bonds. 

Expressed  in  the  scheme  of  the  introduction  U  =  CioHie  in  which 
there  are  two  double  bonds,  and  n  =  10.  During  the  process  of  vul- 
canization alterations  in  adhesiveness,  elasticity,  resistance  to  chemical 
reagents,  solvents,  etc.,  are  therefore  due  essentially  to  a  chemical 
change  in  U,  owing  to  the  addition  of  sulfur  or  sulfur  chloride. 

The  Adsorption  Theory.  —  Wo.  Ostwaldf  has  suggested  that  the 
addition  of  sulfur  is  a  purely  additive  process  similar  to  other  adsorp- 
tion phenomena  where  we  are  certain  that  there  is  no  chemical  com- 
bination. He  offers  the  following  arguments  in  favor  of  this  theory: 

1.  The  addition  of  sulfur  may  be  represented  by  the  adsorption 
equation 

**/  T 

-  =  k  •  cm 
a 

*  Chem.-Ztg.  Jahrg.,  33,  No.  17,  144-5  (1909);  No.  18,  155-7  (1909);  No.  71, 
648-9  (1909);  No.  72,  662-3  (1909). 

t  Kolloidzeit,  6,  136-155  (1910);  10,  146-148  (1912);  11,  38-39  (1912). 


RUBBER 


259 


where  x  is  the  amount  adsorbed,  a  the  amount  of  the  adsorbing  sub- 
stance, c  the  initial  concentration  of  the  substance  adsorbed,  and  k  and 
m  are  constants.  The  equation  also  holds  for  the  addition  of  sulfur 
chloride. 

2.  The  existence  of  a  continuous  series  of  addition  compounds  in 
the  same  mass,  and  formed  at  the  same  time,  is  improbable,  and  is 
unknown  elsewhere  in  chemistry. 

3.  None  of  the  intermediate  members  of  the  series  have  ever  been 
isolated  from  each  other. 

4.  The  apparent  constant  relation  between  sulfur  or  sulfur  chloride 
and  caoutchouc  obtained  by  several  authors  working  with  a  large  ex- 
cess of  the  substance  to  be  adsorbed,  can  be  explained  by  the  fact 
that  they  were  on  the  portion  of  the  curve,  Fig.  39,  where  it  runs 
asymptotically  to  the  abscissae.     In  other  words  they  had  arrived  at  a 


Concentration  of  substance  to  be  adsorbed 

FIG.  39.    Concentration  of  substance  to  be  adsorbed. 

state  of  maximum  adsorption  from  the  standpoint  of  analytical  chem- 
istry, because  further  increases  in  the  adsorbed  sulfur  are  too  small  to 
be  detected. 

C.  O.  Weber  and  later  D.  Spence  and  J.  Young  *  claim  to  have  found 
32  per  cent  to  be  the  maximum  amount  of  sulfur  that  can  become 
fixed.  This  would  correspond  to  the  formula  Ci0Hi6S2.  Moreover 
Hinrichsen  and  Kindscher  obtained  a  body  by  cold  vulcanization  that 
from  analysis  should  have  the  formula  (CioH16)2S2Cl2.  From  this  body 
they  formed  another  substance  by  the  breaking  off  of  hydrochloric 
acid.  The  results  of  the  analysis  would  suggest  the  formula  (CioHi5)2S2. 

5.   The  temperature  coefficient  as  determined  from  the  results  of 
C.  0.  Weber  is  1.8  and  hence  too  low  to  be  that  of  a  chemical  reaction. 
The  latter  vary  from  2  to  3  according  to  van't  Hoff's  rule.    D.  Spence 
and  J.  Young  have  since  found  2.65  to  be  an  experimental  value. 
*  KoUoidzeit,  11,  28-34  (1912). 


260  CHEMISTRY  OF   COLLOIDS 

6.  The  amount  of  sulfur  that  becomes  "fixed"  during  vulcanization 
depends  not  only  upon  the  temperature  and  the  duration  of  the  treat- 
ment, but  also  upon  the  previous  history  of  the  caoutchouc.     This  can- 
not be  explained  on  any  chemical  theory.     On  the  contrary  we  should 
expect  this  peculiar  behavior  if  the  sulfur  is  being  adsorbed  by  the 
caoutchouc,  because  the  molecular  aggregates  of  the  latter  will  vary 
with  the  method  of  preparation. 

7.  Hinrichsen  and  Kindscher  *  have  shown  that  all  the  sulfur  may  be 
extracted  from  cold  vulcanised  rubber,  and  they  claim  to  have  reduced 
the  so-called  combined  sulfur  in  hot  vulcanized  rubber  to  a  very  large 
extent.     The  extraction  follows  a  typical  adsorption  curve,  and  the 
process  of  vulcanisation  is  therefore  a  reversible  one,  although  this 
point  is  not  necessary  for  the  adsorption  theory. 

Objections  to  the  adsorption  theory  have  been  raised  by  several 
authors.  W.  Hinrichsen  has  pointed  out  that  adsorption  phenomena 
are  equilibria  and  can  for  that  reason  be  reached  from  both  sides. 
No  one  has  ever  succeeded  in  completely  extracting  all  the  sulfur  from 
hot  vulcanized  rubber,  consequently  it  is  improbable  that  the  process 
is  reversible.  It  may  be  said  with  regard  to  this  statement  that  it  is 
cumulative  evidence  but  not  a  convincing  argument,  because  a  portion 
of  the  sulfur  may  be  enclosed  by  the  rubber  with  the  result  that 
all  of  it  could  not  be  extracted  without  destroying  the  substance. 
Another  argument  of  Hinrichsen's  seems  well  grounded,  —  to  wit,  ad- 
sorption equilibria  are  quickly  and  easily  reached,  while  the  reaction 
between  sulfur  and  caoutchouc  continues  for  many  hours  even  at  higher 
temperatures.  In  fact  it  has  been  shown  by  Spence  that  all  the  sulfur, 
up  to  10  per  cent  at  least,  would  unite  if  the  process  is  kept  up  for  a 
sufficiently  long  time. 

Hinrichsen  further  remarks  that  other  derivatives  of  caoutchouc  such 
as  the  tetrdbromide  are  known.  Vulcanized  rubber  can  be  changed 
quantitatively  into  this  bromine  derivative  where  the  sulfur  is  almost 
exactly  equivalent  to  the  bromine.  Furthermore  the  changes  in  prop- 
erties during  vulcanization  are  too  fundamental  to  be  explained  on  the 
basis  of  the  adsorption  theory.  He  concludes  from  the  evidence  that 
some  of  the  sulfur,  doubtless  the  free  sulfur,  is  adsorbed,  while  the  re- 
mainder is  combined  chemically. 

D.  Spence  and  J.  R.  Scott  f  have  concluded  from  their  results  that 
sulfur  is  present  in  rubber  in  three  different  states,  free,  adsorbed,  and 
chemically  combined. 

It  may  be  said  in  defence  of  the  chemical  theory  that  it  is  entirely 

*  Kolloidzeit,  6,  202-209  (1910);    10,  146-148  (1912);    11,  38-39  (1912). 
t  Kolloidzeit,  8,  304-312  (1911) 


RUBBER  261 

unnecessary  to  assume  the  existence  of  a  continuous  series  of  addition 
compounds  between  caoutchouc  and  sulfur  or  sulfur  chloride.  More- 
over it  is  also  improbable  that  n  has  any  fixed  value  except  where  a 
definite  chemical  compound  is  formed. 

Regeneration  of  Rubber 

The  problems  involved  in  the  regeneration  of  rubber  are  exactly  op- 
posite in  character  to  those  of  vulcanization.  During  the  latter  proc- 
ess the  .endeavor  is  made  to  render  the  mass  less  plastic,  less  soluble 
in  reagents,  and  more  inert  chemically.  During  regeneration  on  the 
other  hand  the  end  in  view  is  to  re-establish  the  original  properties  of 
the  caoutchouc.  As  sulfur  during  the  vulcanization  process  has  been 
the  cause  of  the  pronounced  alteration  of  properties,  it  is  obvious  that 
regeneration  will  consist  to  a  large  extent  in  the  extraction  of  the  sul- 
fur. We  have  seen,  however,  that  all  the  sulfur  cannot  be  removed 
completely.  This  fact  explains  to  a  large  degree  the  inferiority  of  re- 
claimed rubber  as  compared  to  the  original  substance. 

E.  J.  Fischer  *  has  compiled  a  list  of  the  various  patents  issued  in 
Germany,  France,  England,  and  the  United  States  during  the  last  25 
years,  for  the  recovery  and  regeneration  of  rubber.  A  study  of  these 
patents  reveals  the  fact  that  the  various  methods  are  based  on  three 
salient  factors,  a  rise  in  temperature  (sometimes  under  pressure), 
solution,  and  mechanical  manipulation. 

With  regard  to  the  first  of  these  we  know  that  the  plasticity  of  col- 
loidal substances  may  be  increased  by  a  rise  of  temperature.  This  is 
probably  due  to  a  decrease  in  viscosity  accompanied  by  a  lowering  of 
the  value  of  n.  Doubtless  both  of  these  factors  operate  in  the  case  of 
rubber.  In  practice  the  high  temperature  and  pressure  are  often  ob- 
tained by  the  use  of  steam. 

Solution  also  tends  to  decrease  the  value  of  n.  Any  solvent  that 
attacks  the  sulfur,  the  vulcanized  rubber,  or  the  "filling"  will  tend 
to  increase  the  plasticity.  The  solvents  employed  are  acids,  alkalis, 
naphtha,  benzine,  pyridine,  and  many  others. 

Mechanical  manipulation  tends  to  lower  the  value  of  n  and  there- 
fore increase  the  plasticity.  The  process  may  be  carried  out  during 
the  action  of  the  heat  and  solvents,  or  it  may  be  a  subsequent  treatment. 
*  Gummi-Industrie,  4,  Nos.  1,  2,  and  3. 


CHAPTER  XVI 

TANNING 

The  raw  substance  of  which  leather  is  made  and  the  finished  product 
are  both  colloidal  in  nature.  The  hide  substance  is  a  condensation 
product,  or  products,  of  several  amino  acids.  The  cell  walls  and  the 
cell  contents  are  very  similar  in  chemical  composition,  but  the  latter  are 
much  more  soluble.  The  cell  walls  are,  however,  capable  of  dehy- 
dration, and  also  of  rehydration  to  form  a  gel.  The  cell  substance  is 
doubtless  composed  of  larger  aggregates  than  the  cell  contents.  In 
either  case  the  unit  is  complicated  and  may  be  represented  by  Un, 
where  U  =  Ufnf  +  U"n"  .  .  .  etc.  In  the  cell  substance  n  would 
therefore  have  a  larger  value  than  it  would  in  the  cell  contents. 

The  process  of  tanning  consists  essentially  of  three  steps:  First 
the  rehydration  of  the  partially  dried  hide;  second  the  removal  of  the 
hair  by  the  action  of  alkalis  or  alkali  sulfides;  third  the  precipitation 
of  the  hide  substance  by  the  action  of  tanning  reagents,  such  as  tannin, 
or  the  hydroxides  of  metals. 

Rehydration  and  Swelling.  —  While  water  alone  will  soften  the 
hide  substance,  causing  it  to  swell,  the  reaction  is  much  more  rapid 
in  dilute  acid  or  alkali  solution.  For  the  production  of  soft  thin  leather 
the  hides  are  soaked  in  a  dilute  alkali  solution.  The  action  of  acids 
("pickling"),  such  as  dilute  sulfuric  acid,  is  more  intense,  and  is  re- 
sorted to  where  thick  hides  are  used  to  produce  sole  leather  or  heavy 
belting.  The  swelling  is  entirely  analogous  to  that  of  gelatin,*  and  has 
been  explained  by  some  chemists  in  the  following  manner.  The  cell 
substance,  possessing  both  free  amino  and  carboxyl  groups,  is  an  am- 
photeric  electrolyte,  and  can  therefore  function  as  an  acid  or  as  a  base. 
Although  the  basic  property  is  somewhat  the  stronger,  the  dissociation 
is  very  small  and  the  osmotic  pressure  correspondingly  low.  When 
the  hide  comes  in  contact  with  the  solution  of  acid  or  alkali  a  highly 
dissociated  salt  is  formed  with  the  amphoteric  contents  of  the  cell. 
The  osmotic  pressure  is  greatly  increased  as  a  consequence  and  water 
entering  the  cell  causes  distention  or  swelling. 

The  swelling  process  must  not  be  continued  too  long  else  the  cell 

walls  begin  to  disintegrate  into  a  more  soluble  and  mechanically  weaker 

substance.     The  disintegration  is  greatly  aided  by  the  presence  of  cer- 

*  Koll.  Chem.  Beihefte,  2,  243-284  (1911). 

262 


TANNING  263 

tain  ferments  and  especially  by  the  hydrogen  ion,  hence  there  is  danger 
involved  in  pickling  with  acids. 

The  tanning  proper  is  carried  out  by  the  use  of  tannin,  or  vegetable 
extracts,  or  by  minerals  such  as  the  salts  of  iron,  aluminium,  and  chro- 
mium. The  function  of  the  tanning  agent  is  to  form  an  insoluble  pre- 
cipitate with  the  components  of  the  hide.  This  takes  place  usually  in 
acid  solution.  The  resulting  insoluble  substance,  leather,  is  much  more 
inert  chemically  than  the  hide,  and  as  it  is  insoluble  in  water  there  is 
no  increase  of  osmotic  pressure  and  hence  no  appreciable  swelling. 

Whether  the  combination  of  the  tanning  agent  with  the  hide  sub- 
stance is  purely  chemical,  or  whether  the  reaction  is  to  be  explained  as 
an  adsorption  of  one  substance  by  another,  or  the  coprecipitation  of 
two  colloidal  substances  having  electric  charges  with  opposite  signs, 
is  still  a  controversial  matter.*  The  vegetable  extracts  used  for  tan- 
ning purposes  are  all  colloidal  in  solution.  In  mineral  tanning  a  col- 
loidal gel,  for  example,  iron,  aluminium  or  chromium  hydroxides,  is 
formed  in  the  fibers.  The  reduction  of  bichromate  in  the  fibers  is  aided 
by  reducing  agents. 

Adsorption  undoubtedly  plays  an  important  part  in  the  first  stages 
of  the  tanning  proper.  At  first  considerable  quantities  of  the  tanning 
agent  may  be  washed  out,  while  the  hide  has  not  fully  acquired  the 
properties  of  leather.  As  the  hide  becomes  more  and  more  leather-like 
the  amount  of  the  tanning  agent  that  can  be  extracted  by  water  de- 
creases. This  may  mean  that  secondary  chemical  changes  are  taking 
place,  or  that  the  insoluble  substance  formed  on  the  outside  of  the 
aggregates  is  somewhat  impermeable  to  the  reagent,  and  that  consider- 
able time  is  necessary  for  the  penetration  of  the  tanning  agent  into  the 
center  of  the  aggregate. 

In  practice  gradual  tanning  is  aimed  at.  This  is  accomplished  by 
treating  the  fresh  hide  with  the  spent  liquor,  so  that  the  least  adsorb- 
able  constituents  will  be  adsorbed  first.  The  partially  tanned  hide  is 
then  treated  successively  with  liquors  containing  more  easily  adsorb- 
able  constituents. 

Chromium  salts  are  more  advantageous  for  tanning  than  either  those 
of  iron  or  of  aluminium,  although  aluminium  salts  are  preceptibly  bet- 
ter than  those  of  iron.  The  reason  given  is  that  the  latter  are  hydro- 
lized  too  greatly,  and  the  salt,  therefore,  does  not  so  easily  penetrate  the 
cell  substance.  The  effect  of  iron  salts  can  be  improved  by  the  aid  of 
protective  colloids  such  as  blood,  albumin,  gelatin. 
*  See  Chapter  XI  on  dyestuffs. 


CHAPTER  XVII 
MILK 

Milk  is  a  complicated  colloidal  system*  containing  three  distinct 
disperse  phases:  fat,  casein,  and  albumin.  The  colloidal  properties  of 
albumin  and  casein  have  been  adequately  dealt  with  in  Chapter  XII, 
so  that  for  the  present  purposes  it  will  be  necessary  to  treat  only  the 
fat  constituent  of  milk.  The  considerations  taken  up  here  are  further 
restricted  to  cow's  milk. 

Size  of  Fat  Particles.  —  The  diameter  of  the  fat  particles  differs  be- 
tween the  extremes  0.1  /x  and  22.2  /*.  The  size  is  dependent  upon  many 
factors  such  as  the  breed  of  cattle,  food,  stage  of  milking,  health  of  cow, 
and  length  of  time  since  freshened.  The  average  size  for  Jersey  cattle 
is  3.5  M;  for  Shorthorn  2.92  ^.  The  average  diameter  is  somewhat  larger 
toward  the  end  of  the  flow  than  at  the  beginning  of  the  milking.  The 
particles  diminish  in  size  after  the  cow  has  been  milking  some  months. 

Coagulation  of  Fat  Particles.  —  The  casein  and  the  albumin  act  as 
protective  colloids  for  the  fat  particles,  hence  it  is  necessary  to  offset 
this  protective  effect  before  the  fat  or  butter  can  be  obtained  free  from 
the  casein.  It  is  generally  believed  that  the  casein  forms  an  envelope 
around  the  fat  particles,  but  other  explanations  of  protective  action 
have  been  offered.f 

The  fat  particles  rise  to  the  top  if  the  milk  is  allowed  to  stand,  carry- 
ing part  of  the  casein  with  them.  The  cream  may  also  be  separated 
from  the  major  portion  of  the  liquid  by  centrifugal  motion.  This  is, 
of  course,  the  principle  of  the  cream  separator.  The  fat  may  be  fur- 
ther freed  from  the  casein  by  severe  mechanical  agitation,  called  churn- 
ing in  practice,  but  only  with  great  difficulty  can  this  be  done  if  the 
cream  has  not  become  sour.  The  effect  of  the  mechanical  process  is 
ascribed  to  the  rupturing  of  the  casein  envelopes  by  the  agitation. 
The  fat  being  now  unprotected  rises  to  the  top  and  " gathers"  to  form 
butter. 

The  fat  may  be  separated  quickly  from  the  liquids  if  acid  is  added. 
Small  amounts  of  sulfuric  acid  precipitate  the  casein,  and  the  fat  now 
being  unprotected  comes  out  with  the  casein.     On  the  other  hand  if 
concentrated  sulfuric  acid  is  added  the  casein  is  dissolved,  owing  to  its 
*  Wiegner,  Kolloidzeit,  15,  105  (1914). 
t  Theories  of  Protective  Colloids,  page  111. 
264 


MILK  265 

amphoteric  character,  and  the  fat  separates  out  free  from  other  sub- 
stances. Alkalis  also  dissolve  the  casein  and  hence  allow  the  fat  to 
coagulate.  The  fact  that  butter  is  more  easily  obtained  from  soured 
than  from  sweet  cream  can  be  explained  on  the  ground  that  the  lactic 
acid  formed  destroys  the  protective  action  of  the  casein  by  precipi- 
tating it. 

Homogenization.  —  Fundamentally  this  is  a  process  for  mechanically 
increasing  the  degree  of  dispersion  of  the  fat.  This  is  accomplished  by 
forcing  the  milk,  which  is  heated  to  about  85  degrees,  between  iron  plates. 
The  pressure  sometimes  reaches  250  atmospheres.  The  particles  of 
fat  are  reduced  to  about  one-tenth  their  former  size  and  exhibit  a  vigor- 
ous Brownian  movement. 

Not  only  can  the  fat  particles  already  in  the  milk  be  reduced  in  size, 
but  the  cream  that  has  been  skimmed  off  can  be  redispersed.  More- 
over certain  oils  can  be  substituted  for  the  natural  butter  fat.  It  is 
necessary,  however,  that  casein  be  present  in  the  liquid  in  which  the  fat 
is  to  be  redispersed  else  the  latter  separates  again.  It  is  interesting  to 
note  that  a  very,  much  larger  portion  of  the  casein  is  adsorbed  by  the 
homogenized  fat  than  by  the  particles  in  their  natural  state.  The 
system  is  also  more  stable  for  the  cream  no  longer  rises  to  the  top,  nor 
can  the  fat  be  separated  by  centrifugal  motion.  The  increased  adsorp- 
tion and  the  greater  stability  are  both  in  agreement  with  the  usual 
behavior  of  other  colloidal  systems  when  the  degree  of  dispersion  is 
increased. 

Homogenization  is  employed  to  a  large  extent  in  the  manufacture  of 
ice  cream.  When  the  natural  cream  is  frozen  comparatively  large 
crystals  of  ice  are  formed,  which  give  a  rough  feeling  on  the  tongue. 
In  homogenized  cream  the  number  of  fat  particles  is  about  one  thousand 
times  greater  and  therefore  the  spaces  between  the  particles  are  small. 
As  a  consequence  when  the  homogenized  cream  is  frozen  the  ice  crystals 
are  too  small  to  be  noticeable.  The  product  is  said  to  be  "  smoother. " 
The  white  of  eggs  is  often  added  to  prevent  the  formation  of  large  ice 
crystals. 


CHAPTER  XVIII 
COLLOIDAL  GRAPHITE 

Although  carbon  is  found  in  a  very  fine  state  of  subdivision  in  many 
oils,  coal  tars,  etc.,  the  best  known  example  of  colloidal  carbon  is  "De- 
flocculated  Acheson's  Graphite"*  in  water  or  oil  suspension.  These 
solutions  are  known  technically  as  "Waterdag"  and  "Oildag"  respec- 
tively, names  given  to  them  by  the  inventor,  and  are  used  extensively 
in  lubrication. 

The  preparation  of  these  solutions  was  rendered  possible  by  two 
interesting  observations  made  by  Acheson  in  connection  with  the  graph- 
ite crucible  and  the  carborundum  industries.  He  knew  that  the 
German  clays  used  as  binding  material  in  the  production  of  graphite 
crucibles  possessed  greater  plasticity  than  American  clays  of  similar 
composition.  He  had  also  observed  that  American  clays  taken  some 
distance  from  their  source  were  more  plastic  than  those  lying  near  the 
spot  where  they  had  been  formed  by  the  decomposition  of  the  origi- 
nal rock.  These  two  facts  led  him  to  conclude  that  the  organic  constit- 
uents had  something  to  do  with  the  plasticity  of  clays.  His  second 
observation  bearing  on  this  subject  was  that  carbon  in  an  extremely 
fine  state  of  subdivision  occurred  in  the  furnaces  employed  in  the  manu- 
facture of  carborundum.  He  rightly  concluded  that  this  graphite  was 
formed  by  the  decomposition  of  carbon  compounds  in  the  intense  heat 
of  the  furnace.  It  was  found  that  on  masticating  graphite  formed  in 
this  manner  with  gallotannic  acid  stable  solutions  of  colloidal  graph- 
ite, containing  as  much  as  1  per  cent  of  graphite,  could  be  obtained 
thereby.  In  order  to  produce  an  oil  suspension  it  was  first  necessary 
to  make  a  paste  of  graphite  and  tannin  in  water.  Oil  could  be 
gradually  substituted  for  the  water  during  the  mastication  and  the  oil 
paste  diluted  to  the  desired  consistency. 

The  solutions  thus  formed  behave  in  every  way  like  other  colloidal 
systems.  The  particles  have  a  diameter  of  about  75  ju/z  and  may  be 
precipitated  by  the  action  of  acids.  It  has  not  been  possible  from  the 
literature  to  decide  whether  the  tannin  employed  to  deflocculate  the 
graphite  is  a  true  peptising  agent,  or  only  a  protective  colloid. 

*  Journ.  of  the  Franklin  Instit.,  164,  375  (1907). 


266 


CHAPTER  XIX 
CLAYS  AND  SOILS 

Clays  are  hydrated '  mixtures  of  silicates,  viz.,  iron,  aluminium,  the 
alkalis,  and  alkaline  earths.  Some  sand,  mica,  feldspar,  and  organic 
matter  are  also  usually  present.  The  grains  of  the  constituents  are  more 
or  less  surrounded  by  colloidal  silica,  silicates,  hydroxides  of  iron, 
aluminium  and  manganese,  and  colloidal  organic  matter  such  as  humus. 
The  amount  of  inorganic  colloidal  matter  is  usually  small,  and  varies, 
according  to  Schloesing,*  from  0.5  to  1.5  per  cent.  If  too  little  is  pres- 
ent, the  clay  is  called  weak,  lean,  sandy,  and  has  a  low  plasticity.  On 
the  other  hand  a  large  amount  of  inorganic  colloids  makes  the  clay  fat, 
strong,  and  sometimes  sticky  instead  of  plastic.  A  plasticity  suitable 
for  modeling  or  ceramics,  therefore,  necessitates  a  proper  relation  be- 
tween the  proportions  of  colloidal  and  granular  matter. 

The  effect  of  colloidal  matter  in  clays  is  not  confined  to  plasticity 
alone  for  the  same  constituent  acts  as  a  binder,  and  is  responsible  for 
the  shrinkage  and  cracking  during  firing.  Highly  colloidal  clays 
shrink  more  than  others  and  crack  badly  on  being  dried.  The  incor- 
poration of  granular  matter  increases  the  strength  of  the  ware  by 
reducing  the  shrinkage  and  preventing  to  considerable  extent  the  for- 
mation of  these  cracks.  Frequently  highly  colloidal  clays  are  pre- 
heated before  mixing.  The  effect  of  this  treatment  is  to  dehydrate 
partially  the  colloidal  matter  which  does  not  immediately  become 
rehydrated  on  the  addition  of  water  during  the  mixing. 

This  slow  rehydration  of  clay  is  of  every  day  occurrence,  for  it  is 
very  noticeable  that  after  a  drought  the  soil  does  not  become  sticky 
immediately  following  rain.  On  the  other  hand  the  agriculturalist 
knows  that  clay  lands  become  so  sticky  that  they  are  unworkable  during 
continued  wet  weather. 

The  roads  have  been  greatly  improved  in  some  sections  by  a  proper 
mixing  of  sand  and  clay.  Pure  sand  is  too  loose,  while  pure  clay  be- 
comes too  sticky  during  a  rainy  period.  In  the  proper  proportions  the 
clay  is  the  binder  while  the  sand  produces  the  porosity  necessary  in 
order  that  the  water  may  run  through.  The  sand  also  dilutes  the  clay 
to  the  point  where  the  latter  does  not  become  too  sticky. 

*  Fremy's  Encyclopedic  chimique,  67  (1888). 
267 


268  CHEMISTRY  OF  COLLOIDS 

Adsorption  by  Clays.  —  Owing  to  the  possibility  of  chemical  reactions 
between  the  clay  and  the  adsorbed  substances,  the  phenomena  here  are 
much  more  complicated  than  is  ordinarily  the  case  with  many  colloidal 
systems.  According  to  Sullivan*  changes  between  the  radicals  are 
often  involved.  For  instance  when  acid  or  neutral  salts  are  adsorbed, 
sodium,  potassium,  and  magnesium  from  the  clay  may  be  released  or 
dissolved,  while  an  equivalent  amount  of  the  adsorbed  basic  radical 
remains  with  the  clay.  The  addition  of  alkaline  solution  is  still  more 
complicated.  Not  only  may  there  be  free  alkali  but  basic  solutions 
may  be  formed  because  of  the  hydrolysis  of  salts  of  a  strong  base  and 
a  weak  acid,  e.g.,  carbonates  and  phosphates.  Three  different  reactions 
are  now  possible.  First,  the  free  alkali  may  react  with  the  colloidal 
silica.  Second,  the  silicate  radical  from  the  clay  may  form  insoluble 
salts  with  the  adsorbed  base.  Third,  the  sodium,  potassium,  or  mag- 
nesium displaced  from  the  clay  may  form  soluble  carbonates  and  phos- 
phates, and  these  salts  in  turn  be  adsorbed  by  the  clay  constituents.! 
These  reactions  are  of  great  importance  in  the  study  of  the  fertilization 
of  the  soil.  It  has  been  claimed  that  the  addition  of  lime  not  only 
neutralizes  the  undesirable  acids,  but  also  renders  the  potassium  of  the 
clay  available  for  the  plant. 

The  property  of  adsorption  is  utilized  in  the  determination  of  the 
amount  of  colloidal  matter  in  clays.  Malachite  green  is  often  employed 
for  this  purpose.}  The  basic  radical  of  the  dye  forms  an  insoluble  salt 
with  the  acid  radical  of  the  colloidal  clay.  The  amount  of  dye  taken 
up  is  compared  with  that  adsorbed  by  a  standard  clay  of  known  col- 
loidal content. 

Deflocculation.  —  This  is  a  term  employed  to  indicate  an  increase 
in  the  colloidal  properties,  and  usually  involves  a  lowering  in  the  value 
of  n.  The  principal  defloculants  for  clays  are  alkalis,  or  salts  that  by 
hydrolysis  give  alkaline  reactions.  Chiefly  among  the  latter  are  so- 
dium, potassium,  and  ammonium  carbonates,  oxalates  and  phosphates. 
According  to  Ashley  sodium  carbonate  may  react  as.  follows: 

Na2C03  +  Cagel  -*  CaC03  +  Nagel. 

The  sodium  gel  is  soluble  and  probably  acts  as  a  protective  colloid, 
hence  it  is  possible  to  have  a  suspension  of  clay  formed  that  is  rela- 
tively stable.  However  too  great  concentrations  of  the  deflocculating 
agent  may  cause  recoagulation  and  precipitation  of  the  mixture.  Tan- 
nin acts  as  a  deflocculant  for  clays  containing  iron.  It  appears  to  take 

*  U.  S.  Geol.  Survey,  Bulletin  312  (1907). 

t  Van  Bemmelen,  Landw.  Vers.  Stat.,  23,  267  (1879). 

t  Ashley,  U.  S.  Bureau  of  Standards  Technological  Papers,  23,  40. 


CLAYS  AND  SOILS  269 

the  iron  into  solution  forming  a, soluble,  and  probably  a  protective 
colloid.  Certain  dyes  also  make  good  deflocculants.* 

Soils.  —  Beside  the  constituents  of  the  clay  soil  contains  organic 
colloids,  the  most  important  of  which  is  humus.  This  substance  is 
partly  composed  of  acids,  and  is  formed  as  a  result  of  the  decomposi- 
tion of  organic  matter.  Humus  is  a  good  culture  substance  for  bac- 
teria and  micro-organisms  beneficial  to  the  soil.  Humus  may  also  act 
as  a  protective  colloid  for  other  substances,  but  its  chief  function, 
beside  being  a  depository  of  nitrogenous  matter,  is  to  adsorb  moisture 
and  plant  food,  such  as  nitrates,  potassium  and  ammonium  carbonates, 
and  phosphates.  The  amount  of  moisture  retained  by  the  soil  in  dry 
weather  is  directly  proportional  to  the  percentage  of  colloidal  sub- 
stances; therefore  lean  clays  are  especially  benefited  by  the  presence  of 
humus,  because  they  are  deficient  in  inorganic  colloidal  matter. 

Humus,  being  negatively  charged,  is  coagulated  into  a  gel  by  basic 
substances  such  as  lime.  The  objection  raised  by  some  agriculturalists 
to  adding  lime  and  barnyard  fertilizer  to  the  land  at  the  same  time  seems 
to  be  well  grounded.  The  lime  should  be  added  first  and  allowed  to 
become  thoroughly  air  slaked  (changed  to  carbonate)  before  the  or- 
ganic matter  is  put  on,  otherwise  the  humus  may  be  rendered  too  insolu- 
ble to  function  in  a  manner  beneficial  to  the  soil. 

*  Ashley,  I.  c. 


CHAPTER  XX 

COLLOIDS  IN  SANITATION 

Perhaps  in  no  field  is  colloidal  chemistry  qf  so  much  importance  as 
in  that  of  biology.  A  resume  of  the  work  done  would  require  space 
far  beyond  that  which  is  available  here.  Therefore  a  very  limited  field, 
included  under  the  rather  general  head  of  " Sanitation,"  has  been 
chosen  for  illustration  because  of  its  great  importance  to  both  chemists 
and  biologists.  This  field  involves  those  smallest  of  living  individuals 
known  as  bacteria.  A  bacterium  consists  fundamentally  of  some 
protoplasm  enclosed  in  a  nitrogenous  membrane.  Bacterial  cells  sus- 
pended in  water  or  in  water  solutions  are  colloids.  They  migrate  * 
toward  the  anode  under  the  influence  of  an  electric  current  and  are 
therefore  negatively  charged.  They  are  precipitated  by  ions  of  the 
heavy  metals,  by  aluminium  and  ferric  ions,  and  by  hydrogen  ions,  but 
are  not  so  sensitive  as  the  so-called  suspensoids.f  One  of  the  objects 
of  the  sanitary  purification  of  water  is  the  removal  of  these  bacteria, 
particularly  of  those  which  may  cause  diseases. 

Natural  waters  may  contain  other  colloids  besides  bacteria.  For 
instance  water  collected  from  a  swampy  region  is  usually  colored. 
This  coloring  matter  consists  largely  of  organic  colloidal  material, 
together  with  some  substances  in  true  solution.  The  colloids  are  posi- 
tively charged  except  when  the  water  is  highly  alkaline,  a  condition 
which  probably  causes  the  charge  to  become  negative.  Waters  may 
also  carry  more  or  less  clay  or  silt  which,  if  in  small  enough  particles, 
cause  "  turbidity. "  These  particles  are  negatively  charged.  The  pre- 
cipitation of  colloidal  material  in  water,  either  coloring  matter  or  that 
causing  turbidity,  sometimes  takes  place  under  natural  conditions.  For 
example,  when  a  highly  turbid  stream  meets  another  stream  carrying 
acid  mine  waste  the  hydrogen  ions  in  the  latter  cause  a  precipitation  of 
the  negatively  charged  clay  or  silt  with  the  formation  of  a  deposit  which 
may  become  sufficiently  large  to  affect  the  direction  of  the  stream  flow. 
Muddy  river  waters  on  reaching  the  ocean  may  deposit  suspended 
matter  to  form  a  delta.  This  is  an  instance  of  the  precipitation  of 
colloids  by  salts.  A  colored  water  from  a  swamp  may  enter  a  stream 
containing  colloidal  clay.  The  positive  and  negative  charges  are 
*  Bechhold,  "Die  Kolloide  in  Biologie  und  Medizin,"  p.  189. 
t  Taylor,  "The  Chemistry  of  Colloids,"  p.  304. 
270 


COLLOIDS  IN  SANITATION  .  271 

neutralized  and  partial  precipitation  results,  leaving  in  the  water  an 
excess  of  one  or  the  other  colloid.  For  this  reason  it  is  very  rare  to 
find  a  water  which  is  both  colored  and  turbid,  a  fact  that  is  easily  ex- 
plained on  the  basis  of  colloidal  chemistry. 

The  artificial  purification  of  water  furnishes  an  illustration  of  the 
application  of  colloidal  chemistry  to  industry.  There  are  two  general 
methods  in  use  on  a  large  scale.  The  older,  known  as  slow  sand  fil- 
tration, consists  fundamentally  in  passing  the  water  through  a  bed  of 
sand  on  which  has  formed  a  jelly-like  mass  (called  a  schmutzdecke) 
made  up  of  living  organisms  and  dead  organic  matter  which  have  been 
removed  from  the  water.  Organisms  increase  very  greatly  in  number 
in  schmutzdecke.  This  method  removes  practically  all  the  bacteria 
and  the  turbidity,  but  only  a  portion  of  the  color.  This  removal  de- 
pends on  the  precipitation  of  the  colloids,  first,  by  contact  with  the 
particles  of  sand,  and  second  by  the  colloidal  schmutzdecke.  The 
latter  is  by  far  the  most  important  factor  since  the  filtration  is  not  effi- 
cient until  the  schmutzdecke  is  well  formed.  On  account  of  the  fact 
that  only  a  part  of  the  positively  charged  coloring  matter  is  removed 
while  practically  all  of  the  bacteria  and  other  negatively  charged  col- 
loids are  retained  by  the  filter,  it  is  probable  that  the  jelly-like  mass 
contains  mainly  positively  charged  colloids.  Some  of  the  dissolved 
color  is  undoubtedly  adsorbed  by  the  schmutzdecke  or  by  the  sand. 

The  more  modern  method  of  water  purification  and  one  that  is  ap- 
plicable not  only  to  municipal  supplies  but  also  may  be  used  in  an 
institution  or  a  factory,  is  that  known  as  " mechanical"  or  " rapid" 
filtration,  or  sometimes  as  the  " American  system."  Both  gravity 
and  pressure  filters  are  used  but  the  principle  involved  is  the  same  in 
either  case.  In  this  method  some  coagulant,  generally  aluminium 
sulphate,  sometimes  iron  sulphate,  is  added  to  the  water.  The  alkali 
already  present  in  the  water  or  that  artificially  added,  usually  as  soda 
ash,  causes  a  precipitation  of  the  positively  charged  colloidal  aluminium 
(or  iron)  hydroxide.  The  water  is  then  passed  through  a  basin  in  which 
the  hydroxide  is  coagulated  and  finally  at  a  rapid  rate  through  a  bed 
of  sand.  By  this  process,  practically  all  of  the  turbidity  and  color  are 
removed  together  with  a  very  large  percentage  of  the  bacteria. 

The  method  depends  on  a  number  of  colloidal  reactions.  First, 
there  is  formation  of  a  positively  charged  colloidal  hydroxide.  Second, 
this  hydroxide  reacts  with  the  negatively  charged  bacteria  and  clay 
or  other  colloids.  Third,  the  excess  colloidal  aluminium  hydroxide  is 
precipitated  by  agitation.  Fourth,  the  positively  charged  coloring 
matter  is  precipitated  by  the  negatively  charged  sulphate  ion.  Fifth, 
any  color  in  true  solution  is  adsorbed  by  the  hydroxide. 


272  CHEMISTRY  OF   COLLOIDS 

Catlett  *  has  recently  reported  an  instance  in  which  these  principles 
were  applied  to  solve  a  problem  in  water  nitration.  The  water  was 
unusual  in  that  it  was  both  highly  colored  and  highly  turbid.  It  was  a 
mixture  of  two  waters  which  reached  the  filter  plant  before  sufficient 
time  had  elapsed  for  mutual  precipitation  of  the  colloids.  Alkali  had 
to  be  added  to  precipitate  the  required  amount  of  alum.  In  general 
practice  the  water  is  dosed  with  the  alkali  previous  to  the  addition 
of  the  alum.  In  this  case  poor  results  were  obtained,  the  color  not  being 
completely  removed.  The  trouble  was  remedied  by  adding  an  excess 
of  the  alum  first.  This  gave  an  opportunity  for  the  aluminium  and 
sulfate  ions,  both  of  which  have  high  precipitating  power,  to  act.  Alkali 
was  then  added  to  precipitate  the  hydroxide,  and  the  process  continued 
as  before.  This  change  yielded  a  very  satisfactory  water. 

It  is  known  that  a  water  containing  a  considerable  amount  of  sew- 
age pollution  requires  more  alum  for  purification  than  an  unpolluted 
water  having  the  same  turbidity  and  color.  It  has  been  suggested  that 
this  is  due  to  the  presence  of  protective  colloids  in  the  sewage. 

The  methods  of  sewage  purification  furnish  another  example  of  ap- 
plied colloidal  chemistry.  The  main  object  of  such  purification  at  the 
present' time  is  to  produce  an  effluent  which  can  be  disposed  of  without 
causing  any  nuisance,  in  other  words  a  relatively  stable  effluent.  Ap- 
proximately complete  nitrification  is  also  sometimes  aimed  at,  but  suf- 
ficient stability  can  usually  be  attained  without  this.  The  putrescent 
matter  in  sewage  consists  chiefly  of  nitrogenous  organic  compounds,  a 
large  proportion  of  which  are  present  in  colloidal  solution,  and  are  nega- 
tively charged.  Methods  of  sewage  purification  must  therefore  in- 
volve essentially  the  precipitation  of  this  colloidal  material. 

The  old  method  of  chemical  precipitation,  now  limited  to  the  treat- 
ment of  factory  .wastes,  depends  on  the  same  principles  as  the  rapid 
filtration  of  water  described  above.  The  modern  method  of  intermittent 
sand  filtration,  and  the  still  more  recently  devised  trickling  filters,  are 
of  paramount  importance  to  the  sanitarian.  These  processes  depend  on 
the  precipitation  of  the  colloidal  material  in  the  sewage  by  contact  with 
sand  in  the  first  case  and  with  crushed  stone  in  the  second. f  In  addition 
there  is  also  present  on  the  surface  of  the  stone  a  gelatinous  slimy  growth 
of  micro-organisms,  colloidal  in  nature,  which  is  really  the  essential 
feature  of  the  trickling  filter  process,  and  is  responsible  for  the  precipi- 
tation of  the  colloidal  putrescible  material  in  the  sewage.  The  "col- 
loiders"  devised  at  the  Lawrence  Experiment  Station  are  somewhat 
of  the  same  nature. 

*  Engineering  Record,  73,  741  (1916). 

f  Stein.  Engineering  Record,  69,  525  (1914). 


COLLOIDS  IN  SANITATION  273 

The  most  recent  method  for  sewage  purification,  that  known  as  the 
activated  sludge  process,  is  different  in  principle.  It  was  found  by 
Fowler,  and  more  carefully  studied  by  Ardern  ana  Lockett  in  England, 
by  Bartow  in  this  country  and  by  others,  that  if  a  sample  of  sewage  was 
aerated  for  a  long  period,  several  weeks,  clarification  and  finally  nitri- 
fication took  place.  If  some  of  the  sludge  from  this  settled  sewage 
was  added  to  a  fresh  sample  and  air  applied  again,  the  time  of  puri- 
fication was  greatly  reduced.  Repetition  of  this  process  finally  gave 
satisfactory  results  in  a  few  hours.  The  sludge  used  to  inoculate  a 
fresh  batch  of  sewage  is  known  as  " activated  sludge."  As  far  as  the 
removal  of  colloidal  material  is  concerned,  and  this  is  all  that  is  desired 
in  many  cases,  the  method  has  been  supposed  to  depend  on  mechani- 
cal precipitation.  In  the  opinion  of  the  author,  based  on  the  fact  that 
the  presence  of  activated  sludge  is  essential  even  to  mere  rapid  clari- 
fication, the  process  depends  not  only  on  the  mechanical  effect  on  the 
colloids  but  also  involves  the  precipitating  action  of  micro-organisms, 
or  more  likely  their  enzymes. 

Another  application  of  colloidal  chemistry  is  in  the  study  of  the 
mechanism  of  disinfection.*  The  first  step  involved  consists  in  the 
taking  up  of  the  disinfectant  by  the  bacteria.  This  is  followed  by  a 
poisoning  action  on  the  cell.  As  far  as  we  know  at  present  the  former 
is  the  only  part  with  which  colloidal  chemistry  is  concerned,  for  the 
poisoning  is  probably  a  very  complex  action.  When  bacteria,  which 
can  be  regarded  as  colloids,  are  suspended  in  water  and  a  disinfectant 
added,  the  latter  may  be  distributed  according  to  three  methods,  which 
are  comparable  to  the  action  in  the  taking  up  of  dyes  by  fibres.  There 
may  be  (a)  a  chemical  compound  formed  between  the  disinfectant  and 
the  bacteria,  (6)  a  distribution  according  to  Henry's  law  or  (c)  a  distri- 
bution according  to  the  adsorption  law.  This  latter  is  tested  by  ap- 
plication of  the  adsorption  equation,  x  =  kdn,  where  x  is  the  amount 
of  disinfectant  adsorbed  per  unit  of  adsorbing  substance,  c  is  the  con- 
centration of  the  adsorbed  material  in  the  water  phase,  and  k  and  n 
are  constants.  Comparatively  little  data  is  available  on  which  to 
discuss  the  three  possibilities.  This  is  due  partly  to  the  difficulty  in 
measuring  such  minute  amounts  of  any  disinfectant  removed  by  the 
organisms,  and  partly  to  the  fact  that  many  of  the  best  disinfectants 
are  emulsoids,  which  are  much  more  difficult  to  study  than  are  those 
giving  true  solutions. 

Some  data  has  been  obtained  by  Herzog  and  Betzel  f  using  yeast  cells 

*  Bechhold,  "Die  Kolloide  in  Biologie  und  Medizin,"  pp.  363-379,  also,  Bech- 
hold,  Zeitschr.  f.  Chemie  und  Industrie  der  Kolloide,  6,  22  (1909). 
t  Zeitschr.  f.  physiol.  Chem.,  67,  309  (1910). 


274  CHEMISTRY  OF   COLLOIDS 

with  chloroform,  silver  nitrate,  and  formaldehyde,  and  by  Reichel* 
using  phenol  and  bacteria.  The  conclusions  reached  were  that  formal- 
dehyde formed  a  chemical  compound  with  the  cells,  that  phenol  was 
distributed  according  to  Henry's  law,  and  that  the  taking  up  of  chloro- 
form and  of  silver  nitrate  by  yeast  cells  followed  the  adsorption  law. 
A  study  has  recently  been  made  in  the  author's  laboratory  of  the  dis- 
tribution of  formic  acid  between  water  and  bacterial  cells.  The  ad- 
sorption equation  was  found  to  hold  very  closely.  The  disenfection 
was  shown  to  be  due  to  the  adsorbed  hydrogen  ions,f  the  action  being 
greatly  influenced  by  the  presence  of  salts  in  solution,  but  the  further 
mechanism  of  the  reaction  has  not  been  ascertained.  It  is  firmly  be- 
lieved that  a  more  complete  study  of  disinfection  from  the  colloidal 
standpoint  will  be  productive  of  results  which  are  not  only  of  the- 
oretical interest  but  also  of  great  practical  importance. 

*  Biochem.  Zeitschr.,  pp.  149,  177,  201  (1909). 

t  Norton  &  Hsu,  Journ.  of  Infect.  Diseases,  18,  p.  180  (1916).     The  distribution 
results  have  not  been  published. 


SUBJECT   INDEX 


Absorption,  coefficient,  97. 

See  Adsorption,  57. 
Activation,  of  hydrogen,  125. 

of  sludge,  173. 
Adsorption,  by  charcoal,  57. 

by  meerschaum,  57. 

classification,  60. 

equations,  61. 

isotherms,  54. 

mutual,  113. 

of  colloids,  62. 

of  dissolved  crystalloids,  61,  62. 

of  ions,  48,  49,  54,  55,  57. 

theory  of  vulcanization,  258. 
Agar  agar,  24. 

structure  of  gel,  68. 
Agate,  152. 

Ageing  phenomena,  148. 
Aggregates,  22. 
Agriculture,  29. 
Albumin,  4,  9,  24,  32. 

charge  on,  45,  210. 

concentration  of,  38. 

diffusion  of  egg,  33. 

egg,  26. 

gold,  number  of,  109. 

influence  of  electrolytes  on,  43. 

molecular  weight  of,  33,  39. 

serum,  39. 
Albuminoids,  208. 
Albumoids,  208. 
Albumoses,  38. 

deuteroalbumoses,  38,  39,  213. 

heteroalbumoses,  213. 
gold,  number  of,  108. 

protalbumoses,  38,  39,  213. 
gold,  number  of,  108. 

separation  from  peptones,  212. 

synalbumoses,  gold,  number  of,  108. 
Alcohol,  4,  78,  36. 
Alcogel,  8. 
Alcosol,  8,  120. 


Alizarine  red,  195. 
Alkali  blue,  195. 

Aluminium  oxide  (hydroxide),  170. 
Amicrons,  12,  30. 
Amyloid,  211. 
Aniline  blue,  194,  195. 
Aniline  orange,  195. 
Antimony  sulfide,  45,  178. 
Argentum  Crede,  122. 
Arsenious  sulfide",  4,  24. 
charge  on,  45. 
discharge  by  ions,  51. 
filtration  of,  51. 
Acid,  effect  on  colloids,  25. 
fatty,  3. 
fumaric,  126. 
hydrochloric,  8,  11,  79. 
maleic,  126. 
molybdic,  172. 
nucleinic,  211. 
oleic,  25. 

silicic,  3,  4,  7,  11,  24,  25,  39,  134. 
ageing  of,  148. 
charge  on,  45,  135. 
constitution  of,  68. 
crystallization  of,  136. 
dehydration  of,  138-146. 
dialysis  of,  134. 
gel,  137. 

heat  of  coagulation,  66. 
hydration  of,  147. 
ignition  of,  149. 
irreversible  changes  of,  148. 
occurrence  in  nature,  152. 
organogels  of,  139. 
precipitation  by  electrolytes,  135, 
preparation  of,  134. 
protective  effect,  136. 
solidification  of,  138. 
staining  of,  150. 
structure  of,  140. 
transitions  of,  136. 


275 


276 


SUBJECT  INDEX 


Acid,  stannic,  23,  24,  162. 

a  and  /8,  154. 

charge  on,  45. 

electrolysis  of,  ^7. 

peptisation  of,  155. 

protective  effect,  154. 

theory  of  peptisation,  75,  79. 
stearic,  25. 
titanic,  159. 
tungstic,  172. 
uric,  111. 

Bacteria,  14,  273. 
Barium,  carbonate,  186. 

sulfate,  186. 

Bases,  effect  on  colloids,  25. 
Bavarian  blue,  195. 
Bayrisch  blue,  194. 
Benzidine,  205. 
Benzol,  4. 

Benzo-blue  black,  195. 
Benzo-purple,  22,  46. 
Biebrich  scarlet,  195. 
Bismarck  brown,  104. 
Bismon,  39. 

Bismuth  oxide,  39,  175. 
Blood,  5,  10. 

coagulation  of,  260. 

corpuscles,  10. 
Browian  movement,  19,  20,  40,  41,  252. 

Cadmium,  colloidal,  118. 

sulfide,  45,  178. 
Caesium,  118. 
Calcium,  carbonate  colloidal,  186. 

chloride,  effect  on  colloids,  51. 
Caoutchouc,  4,  5,  251. 

Browian  movement  of  particles,  252. 

charge  on  particles,  252. 

coagulation  of,  253. 

precipitating  agents  for,  254. 

latex,  251. 

size  of  particles,  252. 

source  of,  252. 

vulcanization  of,  256. 
Capillarity  in  gels,  144. 
Capillary  analysis,  205. 
Capri  blue,  194. 
Caramel,  7. 
Carbon  dioxide,  adsorption  of,  59. 


Casein,  32,  236. 

acid  coagulation  of,  237. 

filtration  of,  39. 

gold,  number  of,  237. 

in  milk,  237. 

protective  effect  of,  236. 

rennet  coagulation  of,  237. 
Catalysis  by  platinum  and  palladium, 

114. 

Cataphoresis,  44. 
Cellulose,  32. 
Cement,  6. 

Centrifugalization,  88,  89. 
Ceramics,  6. 
Cerium  oxide,  45. 
Ceri-copper,  175. 
Changes,  irreversible,  5. 
Charcoal,  adsorption  by,  57. 
of  molybdenum  blue,  63 
Charge,  44,  45,  48,  50. 

magnitude  of,  50. 

on  silver,  51. 

theory  of,  71. 
Chlorophyll,  195. 
Chromium  oxide,  45,  171. 
Classification,  in  this  book,  31. 

Noyes',  26. 

Ostwald's,  27. 

other,  3,  19. 
Clay,  colloidal,  24,  267. 
adsorption,  268. 
coagulation  of,  52. 
deflocculation  of,  268. 
suspensions,  22. 
Coagulation,  8,  24,  241. 

by  electrolytes,  51,  52. 

heat  of,  65. 

rate  of,  53. 
Cobalt  oxide,  175. 
Collargol,  122. 
Collodion,  5,  36. 
Colloidation,  at  cathode,  49. 

electrical,  11. 
Colloids,  adsorption  of,  62. 

apparatus  for  determining  migration 
of,  47. 

combinations  of,  80. 

conductivity  of,  78. 

electrical  properties,  44. 

electrolysis  of,  78. 


SUBJECT  INDEX 


277 


Colloids,  freezing  of,  64. 

Graham's  characterization,  6. 

in  medicine,  122. 

inorganic,  32,  86. 

in  urine,  111. 

irreversible,  4,  9,  10,  11,  25. 
stability,  73. 

migration  of,  14,  44. 
colloidal  gold,  103. 

organic,  32,  188. 

precipitation  of,  26. 

mutual  precipitation  of,  55. 

properties  of,  ch.,  111. 

protective,  4,  25,  32,  88,  109,  110. 

reversible,  5,  8,  9,  10,  11,  26. 
stability  of,  72. 

significance  of,  3,  5. 

temperature  effects,  63. 
Color,    change    during    coagulation    of 

gold,  94,  101. 
Colorations,  96. 
Condensers,  cardioid,  14. 

concentric,  14. 

mirror,  14. 
Congo  red,  22,  46,  195. 

diffusibility  of,  195. 

effect  of  electrolytes  on,  197. 

osmotic  pressure  of,  196. 

precipitation  with  colloids,  199. 
Copper,  colloidal,  127. 

ferrocyanide,  179. 
peptisation  of,  80. 

sulfide,  178. 
Cottrell  process,  247. 
Cream,  see  Fat. 

ice,  265. 
Crum,  170. 
Crystal  violet,  204. 
Crystalloids,  3-7,  8,  9,  10. 
Cuvette,  13,  14. 

Definitions,  1,  7. 
Dehydration  of  colloids. 

See  Silicic  acid. 
Devulcanization,  260. 
Dextrin,  4,  7,  25,  32,  39. 

solution  of,  11. 
Dialysis,  7,  1 1,  34. 

apparatus  for,  34. 
Dialyzer  according  to,  Graham,  7. 


Dialyzer  according  to,  Jordis,  34. 

Kuehne,  34. 

Schleicher  and  Schull,  34. 

Zsigmondy  and  Heyer,  34. 
Dichroism  of  gold  particles,  100. 
Dielectric  constant,  44. 
Diffusion,  3,  4. 

constants,  33. 

of  light  rays,  12. 

through  gelatin,  230. 
Disinfection,  273. 
Disperse,  medium,  12,  15,  21,  27,  29,  30. 

phase,  12,  15,27. 

systems,  19,  21,  27,  29. 
Dispersoids,  27. 
Dispersion,  9,  10,  11. 

theory  of  Planck,  102. 
Distension,  227. 

See  Swelling. 
Dyeing,  6,  202. 
Dyestuffs,  25,  32,  193. 

acid,  charge  on,  45,  57. 

basic,  charge  on,  45,  57. 

behavior  toward  electrolytes,  26. 

colloidal  nature  of,  195. 

composition  and  character,  195. 

conductivity  of,  196. 

dialysis  of,  194. 

diffusibility  of,  196. 

influence  of  electrolytes  on,  197. 

influence  of  time  on,  198. 

mutual  precipitation  of,  199. 

osmotic  pressure  of,  196. 

precipitation  of,  198. 

protective  effect  of,  201. 

raising  of  b.p.,  193. 

size  of  molecules,  195. 

ultramicroscopy  of,  194, 

Elastin,  211. 
Electro-endosmosis,  44. 
Electrolytes,  7,  22. 

behavior  toward  colloids,  25. 

See  specific  colloids. 
Emulsoids,  27. 
Emulsions,  27. 
Endosmosis,  44. 
Eosine,  194,  195. 
Equivalence,  79. 
Equivalent,  electro-chemical,  46. 


278 


SUBJECT  INDEX 


Erythrosine,  protective  effect  on  silver 
bromide,  201. 

Fat  particles  in  milk,  10,  236,  264. 

Ferrocyanides,  184. 

Ferments,  Bredig's  inorganic,  87. 

Fibrin,  210. 

Fibrinogen,  210. 

Fibroin,  211. 

Filtration,  mechanical  or  rapid,  271. 

slow  sand,  271. 
Flue  fumes,  243. 

arresting  of,  245. 

centrifugalizing  of,  244. 

electrical  precipitation  of,  247. 

filtering  of,  247. 

settling  chambers,  244. 

washing  of,  244. 
Fluorescein,  35,  194. 
Food,  5,  6. 

Freezing  and  boiling  point  lowering  and 
raising,  33. 

of  colloids,  64. 
Fuchsine,  acid,  195. 

effect  on  colloidal  gold,  104,  194. 

staining  of  SiO2,  150. 

Gamboge,  42. 

charge  on,  45. 
Gases,  centrifugalizing  of,  244. 

washing  of,  244. 
Gelatin,  9,  10,  24,  32,  39. 

decomposition  products  of,  213. 

diffusion  through,  230. 

freezing  of,  65. 

gold  number  of,  110. 

hardening  of,  69. 

influence  of  electrolytes  on,  43. 

osmotic  pressure  of,  224. 

preparation  and  properties  of,  223. 

protective  effect  of,  223. 

reactions  of,  223. 

replacement  of  water  by  other  liquids, 
229. 

solidity  of,  228. 

structure  of,  68,  226. 

swelling  of,  227. 

effects  of  electrolytes  on,  229. 
heat  of,  227. 

ultrafiltration  through,  230. 


Gelatin,  ultramicroscopy  of,  227. 
Gels,  3,  4,  7,  9. 
constitution  of,  68. 
formation  of,  152,  155,  186,  224. 
structure,  of,  69. 

of  silicic  acid  gels,  140. 
Glass,  adsorption  by,  59. 
ruby,  12,  29. 
swelling  of,  29. 
theory  of  formation  of,  31. 
Globin,  32. 
Globulin,  9,  10,  210. 
behavior  of,  222. 
coagulation  of,  39. 
Glue,  5,  7,  9,  24,  25. 
Glycerosol,  120. 
Gold,  colloidal,  4,  11,  26,  89. 
absorption  spectra,  95. 
adsorption  of,  62,  105. 
color   changes   during   coagulation, 

101. 

dichroism  of,  100. 
electrical  migration  of,  103. 
Faraday's,  64. 
filtration  of,  39. 
formation  of  ultramicrons,  95. 
metallic  nature  of,  90,  93. 
nuclear  solution,  91. 
gold  number,  106. 
Paal's,  11. 
pleochroism  of  gold  gelatin  films, 

101. 

polarization  by,  98. 
preparation  of,  89. 
reactions  of,  104. 
size  of  particles,  9. 
theory  of  color,  94. 
Graphite,  coUoidal,  266. 
oil  dag,  266. 
water  dag,  266. 
Gum  arabic,  7,  9,  24,  25. 
Gums,  29. 
Guttapercha,  5,  29. 

Heat  of  colloidal  reactions,  65. 

Hematin,  211. 

Hemoglobin,  4,  5,  11,  24,  32,  211,  231. 

absorption  of  gases  by,  232. 

absorption  spectra  of,  233. 

carbon  monoxide  hemoglobin,  233. 


SUBJECT  INDEX 


279 


Hemoglobin,  diffusion  of,  231. 

iron  content  of,  231. 

molecular  weight  of,  10,  39,  233. 

osmotic  pressure  of,  234. 

pleochroism,  233. 

size  of  molecule,  235. 
Hemolysis,  231. 
Homogenization,  265. 
Humus,  269. 

charge  on,  269. 

coagulation  of,  269. 
Hydrogel,  8. 

See  gels. 

Hydrophane,  152. 
Hydrophiles,  22. 
Hydrophobes,  26. 
Hydrosols,  8,  19,  20,  21. 

behavior  of,  77. 

diffusion  of,  24. 

electrolysis  of,  78. 

evaporation  of,  24. 

irreversible,  10,  22,  24. 
transition  stages  of,  81. 

of  metals,  11,  22. 

of  sulfides,  24. 
Hydroxides,  effect  of  temperature,  64. 

See  Al,  Cd,  iron,  etc. 

Indigo,  195. 

carmine,  195. 
Induline,  194,  195. 
Inoculation,  8. 

lonogens,  ions,  effect  of  valence  on  col- 
loids, 22,  53. 
Iridium,  colloidal,  123. 
Iron,    colloidal   iron    oxide    (hydroxide) 
adsorption  of  arsenious  acid,  169. 

conductivity,  166. 

filtration  of,  39. 

heat  of  coagulation  of,  66. 

hydrogels,  169. 

magneto-optical  properties,  167. 

osmotic  pressure  of,  78,  167. 

rate  of  precipitation  of,  53. 

reactions  of,  163. 

transition  stages  of,  81. 
Isoelectric  point,  49,  50,  73. 

Janus  green,  195. 
precipitation  of,  199. 


Kaolin,  19,  28. 
Keratin,  211. 
Kollagen,  211,  212. 

Lead  oxide,  175. 
Leather,  5. 

tanning  of,  262. 
Litmus,  39. 
Lyophobe,  27. 
Lysalbinnic  acid,  39. 
Lysargin,  39. 

Magdala  red,  194. 

Magnesium,  colloidal  carbonate,  186. 

colloidal,  118. 

Magneto-optical  properties,  167. 
Manganese,  oxide,  175. 
Mastic,  42. 

solutions,  109. 
Medium,  see  Disperse. 
Membranes,  3,  7. 
collodion,  10,  35. 
equilibria,  82. 
fish  bladder,  35.    . 
hydrolytic  decomposition 

of  salts  by,  84. 
parchment,  10,  35. 
preparation  of,  36. 
Mercury,  4,  118. 

colloidal  sulfide,  178. 
Metals,  colloidal,  4,  22,  25,  36. 
according  to  Paal,  123. 
and  ferments,  87. 
charge  on,  86. 
coagulation  of,  52,  86. 
effect  of  electrolytes  on,  26. 
preparation  of,  87. 
protected,  118. 
protection  of,  86. 
stability  of,  88. 
theory  of  coloration  of,  96. 
uses  of,  88. 
Methemoglobin,  232. 
Methylene  blue,  39,  194,  195. 
staining  of  SiO2,  159. 
diffusibility  of,  196. 
Methyl  orange,  electrolysis  of,  47. 
Methyl  violet,  193,  194. 
diffusibility  of,  196. 
staining  of  SiC>2,  150. 


280 


SUBJECT  INDEX 


Micells,  68,  77. 
Microbes,  filtering  of,  35. 
Microscope,  12,  13. 

resolving  limit  of,  19. 

See  Ultramicroscope. 
Migration,  see  Colloids. 
Milk,  10,  236,  264. 

coagulation  of,  264. 

cow's,  237. 

human,  237. 

size  of  fat  particles,  264. 
Mist,  29. 

Molybdenum  blue,  adsorption  by  char- 
coal, 63. 

charge  on,  45. 

colloidal,  128. 
Molybdic  acid,  172. 
Mucines,  211. 
Mucoids,  211. 
Myogen,  216. 
Myosin,  210. 

Naphthalene,  4. 
Neutral  red,  194,  195. 
New  fuchsine,  203. 
Night  blue,  195. 

diffusibility  of,  195. 

osmotic  pressure  of,  196. 
Nigrosine,  195. 
Nile  blue,  194,  195. 
Nonelectrolytes,  effect  of  colloids,  25. 
Nonmetals,  colloidal,  129. 
Nuclein,  236. 

pseudonuclein,  236. 

Oil  emulsions,  21. 

in  water,  20. 
Opal,  152. 
Optical,  constants,  12. 

magneto,  14. 

properties,  11. 
Organogel,  139. 
Organosol,  8,  120. 
Osmium,  123. 
Osmometer,  35. 
Osmosis,  electrical,  44. 

endosmosis,  44. 

cataphoresis,  44. 
Osmotic  pressure,  23,  25,  33. 

of  hydrosols,  36,  43,  78. 


Osmotic  pressure,  and  size  of  particles, 
44. 

effect  of  temperature,  44. 
Oxides,  general,  133. 

See  Specific  metals. 
Oxyhemoglobin,  231. 

crystals  of,  232. 

Palladium,  Paal's  colloidal,  24,  124. 
Pancreas  digestion,  213. 
Parchment,  7. 

Particles,  charge,  on,  48,  49,  51. 
magnitude  of,  50,  51. 

determination  of  size,  15. 

discharge  of,  49,  51. 

motion  of,  40-43. 

number  of,  43. 

rate  of  migration  of,  46,  103. 
Pepsin,  212. 
Peptisation,  8,  11,  24. 

by  colloids,  80. 

of  salts,  80. 

theory  of,  71,  74. 

See  Specific  colloids. 
Peptoids,  161. 
Peptones,  8,  25. 

separation  from  albumins,  212. 
Pharmacy,  6. 
Phase,  see  Disperse  phase. 
Phosphates,  6. 

colloidal  ferric,  179. 
Photography,  6. 
Photohalides,  180. 
Physiology,  6. 
Plants,  6. 
Platinum,  colloidal,  catalytic  effect,  114, 

125. 

charge  on,  45. 
nitration  of  sols,  39. 
poison  effects,  115. 
reduction  by,  125. 
oxides  of  platinum  group,  175. 
Pleochroism,  101. 
Polarization,  12,  15. 

by  gold  solutions,  98. 

plane,  15. 
Potassium,  colloidal,  118. 

carbonate,  6. 

Precipitation,  electrical  (Cottrell   proc- 
ess), 247. 


SUBJECT  INDEX 


281 


Precipitation,  maximum,  57. 

mutual,  55,  57,  199. 

of  flue  fume,  244-247. 

value  of  electrolytes,  54. 
Protalbinnic  acid,  123,  212. 
Protamines,  210. 
Protection  effects,  63. 

theory  of,  111. 
Proteids,  charge  on,  218. 

classification,  209. 

coagulation  of,  221. 

compound,  208. 

denaturization  of,  221. 

glyco-,  211. 

gold  number  of,  106. 

neutral,  217. 

nuclec-,  211. 

precipitation  of,  209,  216,  220. 
fractional,  108. 

separation  of,  110. 

simple,  208. 

solutions,  11. 
Protein,  32. 

bodies,  208. 

decomposition  of,  212. 

osmotic  pressure  of,  213. 
Prussian  blue,  38,  39,  179,  195. 
Pseudonuclein,  236. 
Purple  of  Cassius,  23,  156. 

electrolysis  of,  47. 

peptisation  of,  81. 
Pyrosols,  29,  31. 

Quartz,  solutions  of,  20,  22. 

Radiation  of  gold  solutions,  98. 

Rennet  coagulation,  237. 

Resinates,  4. 

Resins,  32. 

Rubber,  5,  7,  9,  29,  32,  251. 

Caucho,  252. 

Ceara  or  Manicola,  252. 

Para,  253. 

regeneration  of,  261. 

solutions  of,  11,  41. 

ule,  252. 

vulcanization  of,  256. 

See  also  Caoutchouc. 
Rubidium,  118. 

Saltpeter,  6. 


Salts,  colloidal,  25,  32,  179. 
peptisation  of,  80. 

copper  ferrocyanide,  179. 

effect  on  colloids,  25,  26. 

mercurous  halides,  187. 

organic,  32. 

silver  halides,  5,  178,  180. 
Sand,  29. 
Sanitation,  270. 
Scharlack,  194. 
Schwellenwert,  52. 
Sedimentation,  21,  22. 
Selenium,  colloidal,  131. 
Semi-colloids,  25,  173. 
Settling  chambers,  244. 
Sewage,  272. 
Silicates,  colloidal,  29. 
Silicic  acid,  see  Acids. 
Silicon,  128. 
Silk,  5.. 

dyeing  of,  205. 
Silver,  colloidal,  9,  116. 
charge  on,  45. 
crystallization  of,  121. 

cyanide,  179. 

effect  of  surface  on  color,  117. 

effect  of  temperature,  64. 

ferrocyanides,  179. 

halides  of,  179. 

kollargol,  39. 

Lea's,  11,  119. 

lysargin,  39. 

medical  uses  of,  122. 

organosols  of,  120. 

Paal's,  26. 

technical,  122. 
Smoke,  29,  243. 
Soap,  25,  32,  188. 

boiling  point  of  solutions,  188,  190. 

conductivity  of  solutions,  190. 

detergent  effect,  191. 

dialysis  of,  190. 

emulsification  of  fats,  192. 

gel  formation,  189. 

osmotic  pressure  of  solutions,  191. 
Sodium,  colloidal,  4,  186. 

bromide,  186. 

chloride,  4. 
colored,  31. 

iodide,  186. 


282 


SUBJECT  INDEX 


Sodium,  stearate,  3. 
Soil,  5,  267. 
Sol,  8,  39. 

formation,  57. 

See  Colloids. 

Solutions,  colloidal,  4,  5,  8,  9,  21,  26. 
adsorption  of  crystalloids,  62. 
comparison  with  crystalloids,  23. 
conductivity  of,  78. 
Graham's,  21. 
nonaqueous,  186. 
preparation  of,  11. 

crystalloidal,  4,  10,  12,  20,  27. 
Solvents,  3,  4,  8,  9. 
Sorption,  59. 

Sparking,  atomisation,  see  Colloidation. 
Spongin,  211. 
Staining  of  gels,  151. 
Stannic  oxide,  see  Acids. 
Starch,  32. 

charge  on,  45. 

potato,  22. 

soluble,  10,  24. 

structure  of  gel,  68. 
Steam,  29. 
Stearate,  3. 
Subdivisions,  21. 

See  Colloids. 
Sub  microns,  12,  29. 

polarization  by,  31. 
Sugar,  4. 

migration  of,  46. 
Sulfates,  aluminium,  49. 

ammonium,  39. 
Sulfides,  antimonious,  45,  178. 

arsenious,  4,  24,  39,  45,  51. 

cadmium,  45,  178. 

charge,  45. 

colloidal,  25,  32. 

cupric,  45. 

lead,  45. 

precipitation  of,  55. 
Sulfur,  colloidal,  129. 
Surface  tension,  negative,  72. 
Suspensions,  10,  11,  20,  21,  27. 

coarse,  22. 

colloidal,  26. 

filtration  of,  39. 
Suspensions,  suspension  colloids,  26. 

true,  21. 


Suspensoids,  27. 

See  Colloids. 
Swelling  of,  colloids,  9,  24,  25,  43. 

gelatin,  22. 

hides,  2627. 
Syntonins,  212. 

Tanning,  262. 

chrom,  263. 

mineral,  263. 

vegetable,  263. 

Temperature  effects  on  colloids,  63. 
Thorium,  25,  45,  159. 
Tissues,  freezing  of  animal,  65. 

plant,  65. 

Titanium  oxide,  colloidal,  159. 
Transference,  see  Colloids. 
Transitions,  3,  7. 

of  silicic  acid  gels,  141. 
Trypan  red,  195. 
Trypsin,  213. 
Tungstic  acid,  172. 
Tungsten  blue,  74. 

charge  on,  45. 

colloidal,  128. 
Turbidities,  20,  21. 
Tyndall  effect,  12,  15,  23. 

Ultrafiltration,  34,  35,  37,  77,  88,  89. 

through  gelatin,  230. 
Ultramicrons,  adsorption  of  crystalloids, 
62. 

color  of,  100. 

movements  of,  40. 

mutual  adsorption,  63. 
Ultramicroscope,  10,  12,  13. 
Ultramicroscopy,  13,  19. 

See  Specific  colloids. 
Ultraviolet  light,  action  on  colloids,  87. 
Urine,  protective  effect  in,  111. 

Valence  relations,  53. 

of  anion,  54. 
Vanadium  pentoxide,  175. 

charge,  45. 

Violet  black,  194,  195. 
Vitelline,  210. 
Vulcanization,  256. 

•hot  and  cold,  256. 

theories  of,  257. 


SUBJECT  INDEX  283 

Washing  of  gases,  244.  Zeoliths,  122. 

Waste  liquors,  6.  Zirconium  oxide,  25,  45,  128,  159. 

Water,  purification  of  271.  metazirconic  acid,  160. 

Wool,  5. 

dyeing  with  fuchsine,  204. 


INDEX  OF  AUTHORS 


Abegg,  182. 

Acheson,  266. 

Alexander,  J.,  237. 

Amberger,  125. 

Ambronn,  65,  100,  122,  151. 

Applegard  and  Walker,  202. 

Arrhenius,  adsorption  equation,  61,  62. 

Ashley,  269. 

Austin,  244. 

Bachmann,  W.,  142,  226. 

Barratt,  219. 

Barus,  29,  119. 

Bayliss,  W.  M.,  196. 

Bechhold,  37,  39,  56,  112,  198,  230,  270. 

See  Protective  effect,  ultrafiltration. 
Behre,  160. 
Biedermann,  W.,  187. 
Billitzer,  49,  50,  71,  112,  127. 

See  Charge  on  particles. 
Biltz,  W.,  33,  37,  55,  56,  57,  87,  159,  162, 
169,  174,  175,  196,  197,  198,  202. 

See  Precipitation,  law  of  mutual. 
Blake,  93. 
Bodlander,  22,  52. 

See  Schwellenwert. 
Bredig,  G.,  11,  71,  87,  93,  114,  196. 

See  Cblloidation,  charge. 
Brown,  R.,  40. 
Bruhns,  137. 
Bruni,  135. 
Buchner,  G.,  187. 
Burton,  49,  50. 
Butschli,  6,  69,  140,  225,  228. 

See  Gels. 

Campbell,  247. 
Catlett,  272. 
Chevreul,  M.,  191. 
Chrustschoff,  137. 
Cleve,  161. 
Coehn,  44,  163. 


Coehn's  apparatus,  47. 
Coehn  and  Raydt,  45. 
Cohnheim,  209,  219. 
Cotton  and  Mouton,  14,  48,  71,  168. 
See  Ultramicroscope,  migration. 

Debus,  126,  129. 
Ditmar,  252. 
Ditte,  158. 
Doerinckel,  66,  93. 
Donau,  93. 
Donnan,  82,  191. 
Donnan  and  Potts,  192. 
Duclaux,  33,  37,  71,  72,  184. 
Dumanski,  A.,  174. 

Ehrenhaft,  40,  96. 
See  Colorations. 
Einstein,  40. 
Einstein's  formula,  41. 
Elder,  182. 

Falck,  191. 

Faraday,  90. 

Fickendey,  F.,  252,  255. 

Fischer,  E.  J.,  261. 

Fischer  E.  and  Sahlbonn,  206. 

Fischer,  H.  W.,  65,  163,  171. 

Fischer,  M.  H.,  229. 

Fokin,  126. 

Freundlich,  26,  52,  53,  55,  60,  62. 

See  Adsorption. 
Freundlich  and  Losev,  202. 

Galecki,  50,  55. 

See  Migration  of  gold  particles,  103, 
Garnet  Maxwell,  96. 
Georgievics,  G.  von,  202,  205. 
Gerum,  125. 

Gibbs,  Adsorption  formula,  59. 
Goodale,  246. 
Goppelsroeder,  F.,  206. 


285 


286 


INDEX  OF  AUTHORS 


Graham,  3,  7,  9,  10,  25,  33,  34,  55,  66,      Lepkowski,  226. 


139,  152,  161,  175,  179,  230. 
See  Characterization  of  colloids,  6. 
Grimaux,  E.,  134,  175. 
Groschuff,  136. 
Giintz,  182. 
Gutbier,  93,  114,  127. 

Hammersten,  209. 

Hantzsch,  164,  204. 

Hardy,  6,  45,  49,  217,  218,  222,  225. 

Helmholtz,  71. 

Henri,   J.,  252. 

Henri,  V.,  6,  56. 

Herrick,  246. 

Herzog,  33. 

Heyer,  182. 

Hinricksen  and  Kindscher,  260. 

Hober,  R.,  27. 

Hober,  R.  and  Kempner,  194. 

Hofmann,  244,  247. 

Hofmeister,  216,  228,  229. 

Hohlfeld,  247. 

Hoppe-Seyler,  209. 

Hiibl,  V.,  201. 

Hiifner,  37,  233,  234. 

Ignatowsky,  14. 
See  Condenser. 

Jentzsch,  14. 

See  Condenser. 
Jordis,  34,  135. 

See  Dialyzer,  34. 

Kahlenberg  and  Scheiner,  189. 

Kiddie,  244. 

Kirchner,  102. 

Kneckt,  196. 

Kohlschiitter,  117. 

Krafft,  3,  188,  189. 

Krecke,  162. 

Kreuss,  94. 

Kuehne's  dialyzer,  34. 

Kiihne,  W.,  233. 

Kuzel,  128. 

Lea  Carey,  182. 
Lee,  244. 
Lepez,  159. 


Leuze,  127. 

Lewis,  W.  K.,  241. 

Lichwitz,  111. 

Liebermann  and  Bugarsky,  219. 

Liesegang,  152,  230. 

Lillie,  33,  36,  43,  216,  224. 

Lobry  de  Bruyn,  23,  186. 

Lorenz  B.,  31,  183. 

Lottermoser,  8,  44,  56,  122,  127,  158,  172, 

178,  179. 

Luppo-Cramer,  6,  183,  201. 
Luther,  182. 

Majorana,  167. 

Malfitano,  35,  166. 

Marchetti,  174. 

Martin,  247. 

Mayer,  Schaeffer  and  Terroine,  192. 

McBain,  59. 

McBain  and  Taylor,  190. 

Mecklenburg,  155. 

McDougal,  244. 

Menz,  110,  223,  226. 

Michael,  186. 

Michaelis,  193,  194,  222. 

Mie,  96. 

Molisch,  65. 

Mond,  125. 

Moore  and  Parker,  191,  213. 

Moore  an.d  Roaf,  32,  213,  224. 

Much,  111. 

Mueller  and  Pouillet,  58. 

Muller,  A.,  161,  172,  175. 

Mylius,  136. 

Nageli,  6,  68,  169. 

Neisser  and  Friedmann,  56,  198. 

Nell,  230. 

Nernst,  23. 

Neuberg,  186,  187. 

Norton  and  Hsu,  274. 

Noyes,  Arthur  A.,  26. 

Od6n  Sven,  131. 

Ostwald,  W.,  205,  229. 

Ostwald,  Wo.,  27,  167,  215,  232,  258. 

Paal,  39,  43,  123,  127,  212. 
Pappada,  135,  172. 


INDEX  OF  AUTHORS 


287 


Pascheles,  228. 

Pauli,  45,  69,  216,  217,  218,  221,  222,  225. 

Pelet-Jolivet,  207. 

Perrin,  27,  40,  42,  45. 

Pfeffer,  33. 

Picton  and  Linder,  44,  51,  53,  54,  55, 

56,  66,  176,  193. 
Planck,  102. 
Posternak,  217. 
Prange,  68,  119. 
Preyer,  W.,  233. 
Prost,  53. 

Quincke,  6,  21,  40,  44,  138. 

Raehlmann,  11,  194,  195. 

Raffo,  129. 

Ramsay,  125. 

Rayleigh,  15,  97. 

Regnault,  40. 

Reichert,  14. 

Reinitzer,  171. 

Rice,  247. 

Robertson,  237. 

Rodewald,  33,  227. 

Roessing  wire  system,  246. 

Romer,  111. 

Rose,  155. 

Rosenbach,  111. 

Rosenheim  and  Davidsohn,  172. 

Roth,  125. 

Ruer,  160,  164. 

Sabanejeff,  135,  172. 

Sackur  O.  and  Laqueur  E.,  219,  237. 

Salkowsky,  111. 

Schiff,  171. 

Schleicher  and  Schiill,  34. 

Schloesing,  267. 

Schlosing,  22. 

Schmauss,  168. 

Schmidt,  G.  C.,  61. 

Schneider,  64,  119,  152,  178,  179. 

Schoebein,  115,  205. 

Schultz,  108. 

Schulze,  H.,  53,  131,  176,  178. 

Seddig,  42. 

Selmi,  129. 

Senarmont,  137. 

Shelby,  244. 


Shields,  125. 

Siebert,  111. 

Siedentopf,  13,  14,  31,  100, 

Sjoquist,  219. 

Smoluchowski,  40,  41. 

Sobrero,  129. 

Spence,  D.,  255. 

Spence,  D.  and  R.  Scott,  260. 

Spiro,  229. 

Spring,  23,  56,  178. 

Steubing,  97. 

Stoeckl,  120. 

Stoke,  51  (formula). 

Storch,  159. 

Suida,  151. 

Suida  and  Gelmo,  204. 

Sullivan,  268. 

Svedberg,  11,  40,  42,  48,  49,  87,  118,  130. 

Svedberg  and  Snouye,  43. 

Szillard,  161. 

Tammann,  31,  147. 
Teague  and  Buxton,  195,  198. 
Thomsen,  66. 
Tschermak,  151. 

Vanino,  120. 

van  Bemmelen,  6,  58,  62,  142,  169,  175, 

268. 
von  Meyer,  E.,  122,  179. 

Wackenroder,  129. 

Walker,  see  Applegard. 

Weber,  C.  C.,  205,  252,  257. 

Weimarn,  P.,  27,  70,  179,  187,  226. 

Whitney  and  Ober,  51,  176. 

Wiedemann,  44. 

Wiedemann  and  Luedeking,  65,  227. 

Wiegner  C.,  237,  264. 

Wiener,  40. 

Willstatter,  126. 

Winssinger,  C.,  178. 

Wohler,  158,  172,  175,  182. 

Wolfrom,  122. 

Young,  J.,  259. 

Zsigmondy,  R.,  13. 

preparation  of  colloidal  gold,  90. 
theory  of  peptisation,  71. 


288  INDEX  OF  AUTHORS 

Zsigmondy,  R.  and  Heyer,  dialyzer,'  34.  Zsigmondy,  Wilke,  Doerfurt  and  Galecki, 

colloidal  silicic  acid,  135.  37. 

ZsigmonHy  and  Schultz,  fractional  pre-  Zunz,  108. 
cipitation  of  protein,  108. 


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